Bug 16892: Address error checking in comment #47

Bug 16892: Address error checking in comment #47

Tweaked logic to include error check for category
and branch being valid. If not, the user is told
about a configuration error to share with the
library. Otherwise, it should proceed as normal.

TEST PLAN is comment #35 still.
Though include a run with an invalid category
and/or branch code in the two system preferences:
GoogleOpenIDConnectDefaultBranch and/or
GoogleOpenIDConnectDefaultCategory

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>