exception: Report uncaught "Catchable" fatal to "fatal" channel

exception: Report uncaught "Catchable" fatal to "fatal" channel

Things like "Catchable fatal error: Must be X, null given" stop
execution immediately after the error handler callback, and produce
an HTTP 500 Internal Server Error page. They are very fatal.

Per <https://secure.php.net/manual/en/language.errors.php7.php>,
on PHP 7 this results in a TypeError throwable which will
eventually fatal the request and be reported through
set_exception_handler, or be caught by our top-level 'catch'
and artificially forwarded to our set_exception_handler callback.

On HHVM, these fatal error types are PHP5-like in that they
don't have a throwable object to throw yet, instead the error
meta-data is sent directly as parameeters to set_error_handler,
the same as for warnings. We need to intercept them there so
that they are reported correctly.

Sample from PHP 7:

> MediaWiki\Storage\MutableRevisionRecord::newFromParentRevision( null );
> [no req]   TypeError from line 50 of .../MutableRevisionRecord.php:
> Argument 1 passed to ...\MutableRevisionRecord::newFromParentRevision()
> must be an instance of MediaWiki\Storage\RevisionRecord, null given,
> called in /var/www/mediawiki/maintenance/eval.php(78) ...

[exit status: error(255)]

Sample from HHVM:

> MediaWiki\Storage\MutableRevisionRecord::newFromParentRevision( null );
> [hphp] set_error_handler called with:
> ...  $type = 4096 // E_RECOVERABLE_ERROR
>
> [hphp] [...] Catchable fatal error:
> Argument 1 passed to ...\MutableRevisionRecord::newFromParentRevision()
> must be an instance of MediaWiki\Storage\RevisionRecord, null given
> ...

[exit status: error(255)]

Interestingly, if you forget to return false from set_error_handler
for fatal errors on HHVM, it can actually continue execution.

Bug: T205677
Change-Id: I18dd2ff37fa2c2679d0c598cbeff0c61c2fe0253