Create ExceptionHandlerInterface to allow third party exception handlers' to handle fatal errors
feature #10353 [Debug] ExceptionHandlerInterface to allow third party exception handlers to handle fatal errors caught by ErrorHandler (FineWolf)

This PR was merged into the 2.5-dev branch.

Discussion
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[Debug] ExceptionHandlerInterface to allow third party exception handlers to handle fatal errors caught by ErrorHandler

| Q             | A
| ------------- | ---
| Bug fix?      | yes (other project)
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| Related tickets | schmittjoh/JMSDebuggingBundle#68
| License       | MIT
| Doc PR        |

The current `ErrorHandler` is extremely strict on how it selects whether to run an `ExceptionHandler` when an `E_FATAL` occurs.

This modification allows any class that implements `ExceptionHandlerInterface` to handle a `FatalErrorException` created by the `ErrorHandler`.

Commits
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15d063b Create ExceptionHandlerInterface to allow third party exception handlers' to handle fatal errors