Make exception handler comply with HttpKernelInterface

- Don't send response in handleException to comply with HttpKernelInterface
- Added handleUncaughtException which also sends the response
Update Mailer.php

added missing class property
Merge pull request #4075 from janschoenherr/patch-1

Update Mailer.php
Merge pull request #4073 from alprs/fix/exhandler-httpkernel

Make exception handler comply with HttpKernelInterface