Handle non-integer values in fields marked as integers better

George reports:

For a small issue, if you go to the advanced search page and give a non-integer value to fields marked as integers - like orport, dirport, bandwidth or uptime - you get django's debugging page, which fortunately doesn't contain super-sensitive information.

We turned off the Debug mode, so there's no debugging page anymore. But now we only get an empty page in the error case. There should be some output saying that only integers are allowed in those fields. More generally, we should make sure that we check all input very carefully.