Work to fix bug NB#80656.

Work to fix bug NB#80656.
* src/modest-tny-msg.[ch]:
        * Added support for fetching errors in tinymail when we add
	  attachments. In particular, now we set an error in
	  copy_mime_stream if it was not successful.
* src/modest-ui-actions.c:
        * If creating message has errors because of stream read failures
	  (i.e. after removing a media or switching down a server in a vfs
	  stream) we go on but avoid adding the message. We also show an
	  information banner.
* src/modest-mail-operation.c:
        * Manage on message creation the case of an error reading
	  attachments so that we don't stop processing and show proper
	  error at the end.
* src/modest-error.h:
        * Added new error for file io (it'll happen when we fail to
	  retrieve an attachment).

pmo-trunk-r4424