parsemail: Clarify exit codes

parsemail: Clarify exit codes

jk reports that the patchwork error codes are really unhelpful for
correct integration with an MDA. In particular they make sorting out
failures into a separate queue very difficult. Make this better and
clearer: only return 1 on a genuinely unexpected case that requires
adminstrator intervention.

Update the comment for parse_mail regarding return values and exceptions
to line up with how the function actually works and how we use the
results.

Update the tests. None of the existing tests should exit 1; they're
all 'expected' failures: unknown project etc. Also we removed the
exit(0) from the success path, so stop expecting that exception to
be raised.

Add a test for duplicates. That should also succeed without raising
an exception: dups are part of life.

Update parsearchive to deal with the fact that we can no longer
differentiate duplicates.

Reported-by: Jeremy Kerr <jk@ozlabs.org>
Fixes: #171
Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephen@that.guru>