Add _log.debug("session export failed", exc_info=True),
_log.debug("session import failed", exc_info=True), and
_log.debug("session tell failed", exc_info=True) to the DatabaseError
handlers in export_session, import_session, and tell commands respectively.
This brings these handlers into consistency with the existing create, list,
show, and delete handlers which already include the _log.debug call for
diagnostic tracebacks when --log-level debug is used.
Also adds three new BDD scenarios and step implementations that verify
_log.debug is called in each of the three handlers, ensuring the pattern
is tested and cannot regress.
ISSUES CLOSED: #2788
freemo