Fallback to MAPI SubmitMessage if OOM Send fails

* src/common.h (ERR_SEND_FALLBACK_FAILED): New error code.
* src/windowmessages.cpp (gpgol_window_proc): Fallback to SubmitMessage
on OOM Send failure.

This is a workaround for a strange case where the write
in the send event fails. This happens for example if
office documents are attached to a mail. In that
case we can fallback to MAPI as we already do when
Outlooks internal Sign / Encrypt is selected.
Maybe we could be so bold as to always send over
MAPI.

• GnuPG-Bug-Id: T4131