Let commands write errors to STDERR if available (fix issue 2270)

[InvisibleSmiley]

Fixes #2270.

In the end I chose to copy/adapt OutputStyle::getErrorOutput as a new protected method on the abstract command rather than using SymfonyStyle because:

1. creating SymfonyStyle requires both $input and $output, but some existing methods only receive $output
2. SymfonyStyle behaves differently than the plain OutputStyle, which might introduce new issues

I also fixed the exit code for the case where no aliases exist, applying the rule "error output -> exit code 1".