enrol/db/enrol.php ignore delt'd usrs when enroling

enrol/db/enrol.php  ignore delt'd usrs when enroling

Previously the import process would throw a non-fatal error when it found a
non-one number of users with the remote key.  This meant that
when there was a deleted user with the same remote key as an existing user, it
would throw an error, and report that there were no users for that remote key.
Fixed it so that it now ignores deleted users, and provides more accurate
and more precise error messages in the event that there are further
problems.

Author: Peter Bulmer