Prevent fatal error in Special:AbuseFilter/test

When an invalid page name is entered in Special:AbuseFilter/test, display
a sane error message rather than throwing a PHP fatal error.

Bug: 55411
Change-Id: I5ec005180c9009f6d14244f3b03221e525cfaed3
Merge "Prevent fatal error in Special:AbuseFilter/test"