Call sass with --trace

[perlpunk]

Summary

Call sass with the --trace option to get a backtrace in case of an error.

Motivation

Sometimes there are errors like this:

TypeError: String can't be coerced into Integer
  Use --trace for backtrace.

The input is not logged, and not even the line number & file. Hard to debug when the error is not easily reproducible.

References

• Run sass with --trace os-autoinst/os-autoinst-distri-openQA#180

[perlpunk]

A related question: if sass actually fails, how can we react to it? We see the error in the log, but the webserver is started anyway.

[Martchus]

It would probably make sense to abort the startup. Not sure whether sass actually exists with a non-zero return code here.

[perlpunk]

With the trace option we were able to find out the problem. The latest bootstrap css seems to use features that ruby sass isn't prepared for. See https://progress.opensuse.org/issues/162311#note-21 ff.
We added a patch for openSUSE.
Meanwhile I created #151 to support sassc.

[kraih]

Thanks. Don't think we can merge anything before our CI setup is fixed unfortunately.

[kraih]

Rebase on main.

[perlpunk]

Rebased

[kraih]

Seems reasonable.