MDL-64060 message: Use error string identifier instead full message

The conversationdoesntexist string does not exists but it is better to use an error string catchable by an external client.

message/externallib.php

@@ -1707,7 +1707,7 @@ class core_message_external extends external_api {
         } else {
             // We have to throw an exception here because the external functions annoyingly
             // don't accept null to be returned for a single structure.
-            throw new \moodle_exception('Conversation does not exist');
+            throw new \moodle_exception('conversationdoesntexist');
         }
     }
 
@@ -1812,7 +1812,7 @@ class core_message_external extends external_api {
         } else {
             // We have to throw an exception here because the external functions annoyingly
             // don't accept null to be returned for a single structure.
-            throw new \moodle_exception('Conversation does not exist');
+            throw new \moodle_exception('conversationdoesntexist');
         }
     }