avocado.test: Refactor traceback printing code

Reorganize and reduce code repetition by creating functions
to conveniently log/store tracebacks.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.utils.archive: Warn of bad members inside archives

When performing tests with the latest LTP tarball, I found
a bad return from Tarfile().extract_file(), that could be
either a badly formed tarball or a Tarfile() bug. Either
way, the archive module was equipped to deal with such a
situation, not failing the test altogether.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>