4.x - Add pseudo content type for log renderer in ErrorHandler

[adriansuter]

The ErrorHandler is currently instantiating a PlainTextErrorRenderer in the writeToErrorLog-method. So a user would not be able to provide a custom log renderer.

I suggest to add a new pseudo content type "slim-log" to the array of error renderers or to add a new method setLogErrorRenderer() which takes one param ErrorRendererInterface|string|callable.

I prefer the second solution, as the first one might be a little bit hackish.

What are your thoughts?

[RyanNerd]

@adriansuter I've created a PR #2801 for the second option setLogErrorRenderer. Please review. If the logic looks good I'll add unit tests for this feature.