The errors thrown by the send_instant_message WS are not displayed when a teacher sends bulk messages from the course participants page.
Always is shown 'Message sent to X people', even if there is some error.
It has been discovered during
- Now it can be reproduced if a teacher tries to send a message to some student who has blocked him/her.
MDL-63289will get integrated, it will be needed to remove the moodle/course:ignoremessagingpreferences capability to the teacher.