Improved POST and error handling in `be serve`

Improved POST and error handling in `be serve`

POST handling:

Drop the cgi.FieldStorage() in favor of the old urlparse.parse_qs().
We need a dictionary, which FieldStorage is not.  However, I added
.read_post_data() since my old self.rfile.read() was hanging.  The
read_post_data() implementation comes from the
FieldStorage.__init__().

Error handling:

wrap .handle_*() blocks in try/except to handle Storage errors