[bugzilla] Use dont_decode in web.get

lxml refuses to parse unicode strings if the string contains an encoding
decleration. This means the bugzilla module has been broken since the
introduction of the decoding capability of web.get and nobody noticed.

This commit fixes that issue.
[bugzilla] Don't output an error if the bugzilla module was not configured

When the bugzilla module is not configured willie would output an error when
encountering a bugzilla link. This commit fixes this issue.