* Capture Rack's "too many multipart files" error with a spec

* Create a custom error class to handle Rack's multipart error

* fixup! Capture Rack's "too many multipart files" error with a spec

* Use our custom error class when Rack raises a multipart limit error

* Use a Payload Too Large status code for TooManyMultipartFiles errors

* Restore Rack's multipart limit after testing the failure

* Upadate CHANGELOG

* Reword CHANGELOG entry for MultipartPartLimitError change

* Backticks around classname in CHANGELOG

* Change next version of Grape from 1.6.3 to 1.7.0

The introduction of the TooManyMultipartFiles changes API behavior, so bump the
minor version.

* Include the system's configured multipart file limit in the error message

The number of allowed multipart files is a configurable value in Rack, pull that
limit and include it in the generated error message.

* Add a note to UPGRADING about the new TooManyMultipartFiles exception