Refuse a request that cannot be generated with a clear message #68

Background

A request with a zero or negative size gets as far as generation before anything goes wrong. The failure then comes from somewhere deep in the generation code and says nothing useful about what was actually wrong with the request.

Description

Check a request at the entry point, before any generation starts, and refuse it with a message saying what was wrong with it.

A refused request must leave nothing part generated behind.

Testing:

• Assert a request with a size of zero is refused with a clear message
• Assert a request with a negative size is refused with a clear message
• Assert a refused request never starts generating
• Assert a valid request is not refused