exception: Use standard message for HTTP status 500

exception: Use standard message for HTTP status 500

This is the only case in the MediaWiki code base where a custom
message is used. Supporting in the standard HttpStatus library
would require a $customMessage parameter which seems unnecessary.

Additional information should be communicated using additional
HTTP response headers or inside the body content.

Besides, in current PHP versions and in Apache 2.4, the message is
already being ignored and replaced with the standard message (see T100984).

Bug: T100984
Change-Id: I11b58a2f3ce22568479d36027d15e5c6f428ff15