Enhance debug log output for stacktraces

Enhance debug log output for stacktraces

Make MediaWiki\Logger\LegacyLogger and
MediaWiki\Logger\Monolog\LineFormatter better at outputting stacktrace
information and provide support for 'exception' data in the logging
context that is a structured array in addition to the default Exception
object support. This works with MWExceptionHandler::handleFatalError
generated data that is provided by an HHVM interpreter and cannot be
delivered as an Exception class.

With this patch, a good value for LineFormatter's format would be:
"%datetime% %extra.host% %extra.wiki% %channel% %level_name%: %message%
%context% %exception%\n"

Bug: T89169
Bug: T107440
Change-Id: Ida01ed51c573e1654346e716723e543a1be63090