[imp] compose window resets with large attachments

[Chuck Hagenbuch]

Quoting Andrew Morgan <morgan at orst.edu>:

> Now, when I attempt to upload a file too large, the form does not reset
> and I get a notification at the top which says 'Did not attach
> "mylargefile" as the file was empty.'.  I'd prefer something which tells
> the user that the file may have been too large, but this is much better
> than resetting the form.

We have the better checks for this and other errors in HEAD - it'll be  
in IMP 4.2.

-chuck