Your suggestion could probably be applied as a bug fix to maintenance releases, but is it worth the effort?  We don't generally try to protect programmers from themselves in Python.

On the other hand, there should clearly be a way to provide the 'explain' text as well as the message text.

In 3.4 we could add an 'explain' keyword to send_error.  If only explain is specified, we'd use the shortmsg for message.  That way, a programmer writing new code can make it work the way they want it to, while existing code will continue to work the way it used to.