MDL-68070 messaging: Ensure that error is shown on first messages
Before there is a conversation, messages between users are sent
using core_message_send_instant_messages instead of
core_message_send_messages_to_conversation.
That method had an undefined $errormessage variable, ultimately
causing the problem detected on message_post_message()/message_send()
to never been returned by the external.
This just adds the same error than already existing conversations do,
but returning it as warning, for BC, JS will show it.