ShellCommandResult: Fix doctest
ShellCommandResult: Fix doctest

The doctest uses python3 syntax while invoking `python`,
which fails if the executable `python` is the Python 2 runtime.

Also check stderr before stdout, as it will show why stdout
is not as expected.

Fixes https://github.com/coala/coala/issues/3780

ayushin78/coala,netman92/coala,coala-analyzer/coala,refeed/coala,JohnS-01/coala,coala/coala,CruiseDevice/coala,karansingh1559/coala,rimacone/testing2,yashLadha/coala,Shade5/coala,shreyans800755/coala,Nosferatul/coala,arjunsinghy96/coala,CruiseDevice/coala,ayushin78/coala,kartikeys98/coala,aptrishu/coala,damngamerz/coala,Shade5/coala,ayushin78/coala,Shade5/coala,incorrectusername/coala,SanketDG/coala,shreyans800755/coala,coala-analyzer/coala,refeed/coala,jayvdb/coala,aptrishu/coala,JohnS-01/coala,Asnelchristian/coala,aptrishu/coala,incorrectusername/coala,coala/coala,Balaji2198/coala,Balaji2198/coala,arush0311/coala,Nosferatul/coala,damngamerz/coala,sagark123/coala,Asalle/coala,nemaniarjun/coala,RJ722/coala,arush0311/coala,kartikeys98/coala,SanketDG/coala,shreyans800755/coala,arjunsinghy96/coala,Asnelchristian/coala,Balaji2198/coala,RJ722/coala,damngamerz/coala,Nosferatul/coala,MariosPanag/coala,rimacone/testing2,sagark123/coala,netman92/coala,coala-analyzer/coala,incorrectusername/coala,rimacone/testing2,MariosPanag/coala,CruiseDevice/coala,jayvdb/coala,refeed/coala,RJ722/coala,yashLadha/coala,yashLadha/coala,SanketDG/coala,karansingh1559/coala,JohnS-01/coala,netman92/coala,kartikeys98/coala,nemaniarjun/coala,coala/coala,karansingh1559/coala,nemaniarjun/coala,Asalle/coala,arjunsinghy96/coala,Asalle/coala,Asnelchristian/coala,arush0311/coala,jayvdb/coala,MariosPanag/coala,sagark123/coala