'glance-manage db purge' command fails with DBReferenceError due
to FK constraint failure and exits with stack-trace on the command
prompt.
Made changes to give user-friendly error message to the user as
well as log appropriate error message in glance-manage logs
instead of stack-trace.
Co-author-by: Dinesh Bhor <dinesh.bhor@nttdata.com>
Change-Id: I52e56b69f1b78408018c837d71d75c6df3df9e71
Closes-Bug: #1554412