improve exception handling / diagnosis

[ThomasWaldmann]

in attic.archiver:727:

try:
    exit_code = archiver.run(sys.argv[1:])
except Error as e:
    archiver.print_error(e.get_message())
    exit_code = e.exit_code

The problem here is that print_error only prints very little information. No traceback, thus no information where it crashed. For debugging, I just added a "raise" before this line, to re-raise the Error and get a normal traceback.

[ThomasWaldmann]

Here's some patch to show tracebacks:

https://github.com/ThomasWaldmann/attic/commit/14d91a25fcfc68a16cfb29e8eb0ab203d4d236c0

Less pretty, but more useful in case of errors / hangs.