Don't print tracebacks for expected errors

[nirs]

ovirt-img print tracebacks for all errors, however some errors are expected and a traceback is not helpful.

• Engine login error (bad username or password, user error)
• Downloading non-existing disk
• Downloading locked disk (e.g. used by running vm, or another storage operation)
• Error creating the target image (e.g. permission issue)
• Uploading non-existing image (user error)
• Uploading to non-existing storage domain (user error)
• Uploading when user is not allowed to create disk (engine permissions issue)

In general traceback should be logged only for unexpected error, which may be a bug in the tool or some environment issue. In this case the traceback can help debugging the issue.

[nirs]

One case (termination by signals) is handled in #116.

We can have similar mechanism, using common error class for
expected errors and handling them in _tools.main().