core: Include tracebacks in QueriesCommandBase errors.

At present DEBUG logging is required to view a full trace
of exceptions caught by the QueriesCommandBase class. These
really should be present by default within the ERROR line.

Change-Id: I48fa907c9e6cf7365768f90b2dd90d167114f051
Signed-off-by: Lee Yarwood <lyarwood@redhat.com>