BadAmount exception not handled

[zbynekwinkler]

https://app.getsentry.com/gittip/gittip/group/7623582/

Stacktrace (most recent call last):

  File "aspen/website.py", line 76, in handle_safely
    response = self.handle(request)
  File "aspen/website.py", line 109, in handle
    response = request.resource.respond(request)
  File "aspen/resources/dynamic_resource.py", line 47, in respond
    exec self.pages[1] in context
  File "/app/www/%username/tip.json.spt", line 31, in <module>
    , body['amount']
  File "gittip/models/participant.py", line 326, in set_tip_to
    raise BadAmount

[zbynekwinkler]

This groups with #1521.