Source not available of user programs does a os.chdir

[skogsbaer]

See the discussion here: #22

Upshot:

• Error messages should contain relative file names (to avoid clutter)
• If the program being run, the file names should be relative to the directory from which the program run started. Nevertheless, loading the source code should succeed.

[CodeSteak]

(fix in #26 was reverted, it didn't work for "normal" traces)