EBookBackendSqliteDB: various fixes to GError handling
EBookBackendSqliteDB: various fixes to GError handling

At different points in the backend, GErrors were not being thoroughly
checked before being handed off to further calls that might set/propagate
them, which can lead to internal glib warnings/errors.

Therefore, in all instances that GErrors might be re-used within the
scope of a single function call, add explicit checks and ensure that
the codepath behaves appropriately with regards to error status and
memory cleanup.

Signed-off-by: Sean Finney <e20ccb4a048643b1e06e2af9add624af020321e3@seanius.net>

GNOME/evolution-ews,GNOME/evolution-ews