Error handler 2.1.0

Error handler package was updated with the following enhancements:

• There is now Yiisoft\ErrorHandler\Event\ApplicationError triggered after the error is handled.
• Current working directory is restored in error handler so relative paths now work well.
• Exit is deferred now so custom shutdown handlers are properly triggered.
• Added markdown support for friendly exceptions.
• Added support for psr/log 2.0 and 3.0.