Process not successful exception message

If not isSuccessful but getErrorOutput is empty, at least exception message is process->getOutput
Merge branch 'patch-1' of https://github.com/squaini/CompassElephant into squaini-patch-1

Conflicts:
	src/CompassElephant/CommandCaller.php
unused use statement
Merge branch 'squaini-patch-1' into develop