MWExceptionHandler: Do not use 'exception' for custom log data

When we create an exception-like set of data for logging in
MWExceptionHandler::handleFatalError, add it to the logging context as
a 'fatal_exception' member. The 'exception' value in a log context
should only be populated with a real Exception object.

Bug: T150106
Change-Id: I253943849f19ed5480dbda7bfbc0bf607f69c47d

includes/exception/MWExceptionHandler.php

@@ -284,7 +284,7 @@ TXT;
 		$trace = $trace ?: debug_backtrace();
 		$logger = LoggerFactory::getInstance( 'fatal' );
 		$logger->error( $msg, [
-			'exception' => [
+			'fatal_exception' => [
 				'class' => 'ErrorException',
 				'message' => "PHP Fatal Error: {$message}",
 				'code' => $level,