pluginErrorHandler() displays all errors in debugging mode.

Even strict and deprecation notices, and any future error
levels. Shows the error level code instead of the label
if it runs into an uncommon type.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5423 2fea6a4d-a838-0410-917b-93a53c48e9d2