ErrorHandler and fixes
merged branch uuf6429/master (PR #8170)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #8170).

Discussion
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ErrorHandler and $context fixes

Fixes a minor issue with optional parameters not being optional and added support for `ContextErrorException` class as detailed here: https://github.com/fabpot/Silex/issues/717

Commits
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784726d ErrorHandler and  fixes