Notification of string changes

Author: Sara Arjona
MDL-63901 core_message: Fix error when the conversation does not exist
Create a conversation instead of throwing an exception to get the
behaviour working as it was before.
The string lang 'conversationdoesntexist' can be removed because
it was added in MDL-63466 and it's not used anywhere more.


http://git.moodle.org/gw?p=moodle.git;a=commit;h=d9552c3c33d4b3d7344da386ae35206a6a464e15
http://github.com/moodle/moodle/commit/d9552c3c33d4b3d7344da386ae35206a6a464e15

- 3.6 en [conversationdoesntexist,core_message]