execute_bear: Improve ``AssertionError`` message
execute_bear: Improve ``AssertionError`` message

Improve AssertionError message so if ``msg`` is maybe really empty,
we don't have a superfluous ``\n``.

Closes https://github.com/coala/coala/issues/4418

SanketDG/coala,damngamerz/coala,rimacone/testing2,shreyans800755/coala,shreyans800755/coala,aptrishu/coala,Asalle/coala,Asalle/coala,jayvdb/coala,coala/coala,SanketDG/coala,aptrishu/coala,CruiseDevice/coala,karansingh1559/coala,Nosferatul/coala,coala/coala,kartikeys98/coala,nemaniarjun/coala,aptrishu/coala,jayvdb/coala,shreyans800755/coala,Nosferatul/coala,jayvdb/coala,kartikeys98/coala,Nosferatul/coala,nemaniarjun/coala,kartikeys98/coala,SanketDG/coala,coala-analyzer/coala,coala-analyzer/coala,CruiseDevice/coala,damngamerz/coala,karansingh1559/coala,rimacone/testing2,damngamerz/coala,Asalle/coala,coala/coala,rimacone/testing2,CruiseDevice/coala,nemaniarjun/coala,coala-analyzer/coala,karansingh1559/coala