Only add stdout handler when running as cli
Add one more test
Catch exceptions with decorator

This allows us to validate the exact output in the tests
Don't use click's newline echoing

click.echo does two write calls, which get flushed immediately. Using
this in multiprocessing might make output overlap. Using a single
write-call apparently resolves that issue, although i don't know why.

A proper solution would be to use locks, but e.g. multiprocessing's
logger doesn't seem have nearly as much flexibility as others. For now
it seems to be bugfree enough.
More tests