Make hookErrorHandler() only care about serious signature errors

Make hookErrorHandler() only care about serious signature errors

Previously, it would send all manor of warnings to the error
and error-json channels. This adds a lot of overhead due to
AbuseFilter parse/eval errors. By passing immediately instead
of after calling handleError(), that overhead is avoided. Since
it still passes (e.g. returns false), any default PHP warning
logging still applies.

Change-Id: I18e60c09c2a48f2e26abab5d451bb52ea4ba7961