feature #27499 Improved an error message related to controllers (javiereguiluz)
feature #27499 Improved an error message related to controllers (javiereguiluz)

This PR was squashed before being merged into the 4.2-dev branch (closes #27499).

Discussion
----------

Improved an error message related to controllers

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This proposal is irrelevant for experienced users but it may be useful for newcomers. After having delivered several introductory Symfony training, I can say that when someone adds `return "some string...";` in their controller, the error message is confusing:

![before](https://user-images.githubusercontent.com/73419/40959468-0faf790a-689d-11e8-9ce1-f6e0caf4b113.png)

Maybe we can reword it a bit? (I'm open for suggestions to improve the error message)

![after](https://user-images.githubusercontent.com/73419/40959505-29747070-689d-11e8-834e-92bf18760469.png)

Commits
-------

7510c3a335 Improved an error message related to controllers

gimler/symfony,ro0NL/symfony,andrerom/symfony,Einenlum/symfony,pyrech/symfony,garak/symfony,docteurklein/symfony,arjenm/symfony,romaricdrigon/symfony,zcodes/symfony,joelwurtz/symfony,andrerom/symfony,Taluu/symfony,mpdude/symfony,pamil/symfony,dmaicher/symfony,Slamdunk/symfony,tucksaun/symfony,docteurklein/symfony,garak/symfony,pierredup/symfony,fabpot/symfony,fabpot/symfony,pamil/symfony,pyrech/symfony,andrerom/symfony,Simperfit/symfony,yceruto/symfony,sroze/symfony,Simperfit/symfony,Einenlum/symfony,tucksaun/symfony,zerkms/symfony,tgalopin/symfony,hhamon/symfony,e-moe/symfony,HeahDude/symfony,andreia/symfony,paradajozsef/symfony,sroze/symfony,OskarStark/symfony,chalasr/symfony,deguif/symfony,jakzal/symfony,ro0NL/symfony,vincentaubert/symfony,jderusse/symfony,jakzal/symfony,ogizanagi/symfony,paradajozsef/symfony,mweimerskirch/symfony,curry684/symfony,lstrojny/symfony,Deamon/symfony,fabpot/symfony,mnapoli/symfony,garak/symfony,bendavies/symfony,derrabus/symfony,e-moe/symfony,michaelperrin/symfony,Taluu/symfony,xabbuh/symfony,yceruto/symfony,Nyholm/symfony,pamil/symfony,Koc/symfony,tucksaun/symfony,weaverryan/symfony,smatyas/symfony,hhamon/symfony,mweimerskirch/symfony,curry684/symfony,Deamon/symfony,stof/symfony,symfony/symfony,teohhanhui/symfony,sgehrig/symfony,deguif/symfony,paradajozsef/symfony,ro0NL/symfony,ogizanagi/symfony,sgehrig/symfony,kbond/symfony,jakzal/symfony,voronkovich/symfony,chalasr/symfony,weaverryan/symfony,mnapoli/symfony,Tobion/symfony,zcodes/symfony,jderusse/symfony,1ed/symfony,bendavies/symfony,localheinz/symfony,romaricdrigon/symfony,zcodes/symfony,oleg-andreyev/symfony,HeahDude/symfony,derrabus/symfony,e-moe/symfony,Tobion/symfony,kbond/symfony,localheinz/symfony,zcodes/symfony,vincentaubert/symfony,hhamon/symfony,1ed/symfony,bendavies/symfony,bendavies/symfony,nicolas-grekas/symfony,garak/symfony,dmaicher/symfony,Deamon/symfony,e-moe/symfony,voronkovich/symfony,pierredup/symfony,Deamon/symfony,paradajozsef/symfony,oleg-andreyev/symfony,andreia/symfony,jakzal/symfony,docteurklein/symfony,damienalexandre/symfony,mpdude/symfony,andreia/symfony,hhamon/symfony,tgalopin/symfony,mnapoli/symfony,sroze/symfony,gonzalovilaseca/symfony,localheinz/symfony,gonzalovilaseca/symfony,Slamdunk/symfony,gnat42/symfony,Slamdunk/symfony,mpdude/symfony,Koc/symfony,symfony/symfony,yceruto/symfony,chalasr/symfony,mpdude/symfony,jvasseur/symfony,mweimerskirch/symfony,gonzalovilaseca/symfony,vincentaubert/symfony,ogizanagi/symfony,romaricdrigon/symfony,nicolas-grekas/symfony,dmaicher/symfony,HeahDude/symfony,arjenm/symfony,docteurklein/symfony,sgehrig/symfony,Nyholm/symfony,ro0NL/symfony,gonzalovilaseca/symfony,joelwurtz/symfony,smatyas/symfony,curry684/symfony,jvasseur/symfony,derrabus/symfony,maidmaid/symfony,andreia/symfony,derrabus/symfony,deguif/symfony,shieldo/symfony,alekitto/symfony,Nyholm/symfony,Tobion/symfony,smatyas/symfony,oleg-andreyev/symfony,pierredup/symfony,mnapoli/symfony,Deamon/symfony,Simperfit/symfony,frankdejonge/symfony,Einenlum/symfony,pyrech/symfony,teohhanhui/symfony,frankdejonge/symfony,damienalexandre/symfony,tgalopin/symfony,damienalexandre/symfony,Taluu/symfony,pamil/symfony,chalasr/symfony,zorn-v/symfony,phramz/symfony,stof/symfony,lyrixx/symfony,ragboyjr/symfony,phramz/symfony,phramz/symfony,alekitto/symfony,jvasseur/symfony,weaverryan/symfony,voronkovich/symfony,deguif/symfony,xabbuh/symfony,Einenlum/symfony,shieldo/symfony,d-ph/symfony,voronkovich/symfony,michaelperrin/symfony,teohhanhui/symfony,damienalexandre/symfony,shieldo/symfony,arjenm/symfony,jvasseur/symfony,nicolas-grekas/symfony,sgehrig/symfony,symfony/symfony,lstrojny/symfony,zorn-v/symfony,Simperfit/symfony,ragboyjr/symfony,Tobion/symfony,smatyas/symfony,weaverryan/symfony,zorn-v/symfony,arjenm/symfony,kbond/symfony,d-ph/symfony,Nyholm/symfony,1ed/symfony,joelwurtz/symfony,zerkms/symfony,michaelperrin/symfony,OskarStark/symfony,oleg-andreyev/symfony,michaelperrin/symfony,kbond/symfony,1ed/symfony,MatTheCat/symfony,yceruto/symfony,gnat42/symfony,jderusse/symfony,lyrixx/symfony,jderusse/symfony,ragboyjr/symfony,xabbuh/symfony,lstrojny/symfony,mnapoli/symfony,phramz/symfony,OskarStark/symfony,ragboyjr/symfony,stof/symfony,d-ph/symfony,zorn-v/symfony,MatTheCat/symfony,d-ph/symfony,maidmaid/symfony,fabpot/symfony,ajgarlag/symfony,xabbuh/symfony,smatyas/symfony,localheinz/symfony,Taluu/symfony,zerkms/symfony,alekitto/symfony,curry684/symfony,Koc/symfony,maidmaid/symfony,gnat42/symfony,Simperfit/symfony,zerkms/symfony,MatTheCat/symfony,tgalopin/symfony,gimler/symfony,joelwurtz/symfony,mweimerskirch/symfony,gimler/symfony,ajgarlag/symfony,andrerom/symfony,maidmaid/symfony,romaricdrigon/symfony,tucksaun/symfony,Koc/symfony,vincentaubert/symfony,gimler/symfony,stof/symfony,pierredup/symfony,shieldo/symfony,nicolas-grekas/symfony,mweimerskirch/symfony,frankdejonge/symfony,dmaicher/symfony,ajgarlag/symfony,teohhanhui/symfony,symfony/symfony,frankdejonge/symfony,lyrixx/symfony,lyrixx/symfony,OskarStark/symfony,MatTheCat/symfony,alekitto/symfony,HeahDude/symfony,gnat42/symfony,ajgarlag/symfony,docteurklein/symfony,ogizanagi/symfony,Slamdunk/symfony,sroze/symfony,pyrech/symfony