Improve error handling in imapx_command_fetch_message_done().
Improve error handling in imapx_command_fetch_message_done().

Spotted a few execution paths where we claim success but have no
stream object for the fetched message.

Distrotech/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,Distrotech/evolution-data-server,Distrotech/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,Distrotech/evolution-data-server,Distrotech/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server