Return E_FAIL when cancelling write event

Return E_FAIL when cancelling write event

* src/mailitem-events.cpp (Write): Return E_FAIL to cancel write
event.

--
This is intended to prevent other Addins from changing the return
code of the event to their value as after the E_FAIL no other
addin should be called.

GnuPG-Bug-Id: T6125