Ember: return HTTP 431 when maxHeaderSize is exceeded

[rossabaker]

• Adds a maxHeaderSizeErrorHandler property to EmberServerBuilder
• The default returns an empty 431 response when the default message length is exceeded. Currently, we return a 500.

I don't love the proliferation of options, but I think this is in the original spirit of the errorHandler. This needs a rethink in 1.0.

Fixes #7395.

[danicheg]

... Anyway, in the current design, this is perhaps the only way to embed error handling properly.

[rossabaker]

The simpler way is just to add a case to the default errorHandler, but even the example apps obliterate the default instead of composing it. 🤷