web interface: make maintainer count and coverage filters smaller
web interface: minor cleanup of AJAX request handling
package updater: when downloading tarballs check that we don't get HTML error message.

Many web servers have misconfigured error handling, and return 200 OK code with
error message instead of 404. This is a check that no HTML or plaintext is
returned for something that obviously should be an archive.
package updater: handle CPAN tarballs moving around