Notification of string changes

Author: Sara Arjona
MDL-78102 badges: Check empty backpack email/password
For backpack connection using OBv2.0, email and password can't be
blank.
This patch adds some extra checks to validate this and displays an
error to the users before trying to connect to the backpack.
Besides, the error displayed when any error is returned by the
backpack (like invalid credentials), has also been improved, to
make it clearer for the users.


http://git.moodle.org/gw?p=moodle.git;a=commit;h=7e74ec4a3c634a1e452ee2fc8c9601708efd0983
http://github.com/moodle/moodle/commit/7e74ec4a3c634a1e452ee2fc8c9601708efd0983

+ 4.1 en [backpackconnectionunexpectedresult,core_badges]
+ 4.1 en [backpackemail_required,core_badges]
+ 4.1 en [password_required,core_badges]