Improved exception message.
Improved exception message.

Before: ```Unknown message type "johndoe". Available are [foobar].```

After: ```Unknown message type "johndoe". Available are [foo,bar].```
jderusse/SwarrotBundle,swarrot/SwarrotBundle,atailouloute/SwarrotBundle,odolbeau/SwarrotBundle,olaurendeau/SwarrotBundle,atailouloute/SwarrotBundle,jderusse/SwarrotBundle,glutamatt/SwarrotBundle,atailouloute/SwarrotBundle,olaurendeau/SwarrotBundle,stof/SwarrotBundle,metfan/SwarrotBundle,lafourchette/SwarrotBundle,lafourchette/SwarrotBundle,odolbeau/SwarrotBundle,glutamatt/SwarrotBundle,jderusse/SwarrotBundle,olaurendeau/SwarrotBundle,odolbeau/SwarrotBundle,lafourchette/SwarrotBundle