[IMP] mail: don't clear undelivered chatter message

When a message is sent on the chatter and the call fails (for example
because of an internet connection failure), the message was still
cleared so the content was lost.

This change keep the same behavior for chat window and discuss, but
change the behavior in a chatter so the message is only cleared if the
rpc has passed.

closes #14759
opw-696971