• be explicit about how tuples are unpacked
• allow bytes for status value
• allow Headers for headers value
• use TypeError instead of ValueError
• errors are more descriptive
• document that view must not return None
• update documentation about return values
• test more response types
• test error messages

closes #1676

TypeError is more correct by definition (wrong type to function), and I don't think anyone should be catching that error anyway (what would it mean to do that?), so I'm ok if this somehow breaks some code. This is also consistent with what force_type returns for bad input.

I had to get a bit creative with displaying the error from force_type while injecting a new descriptive message. It would be nice if we could just use raise new_error from original_error, but that's Python 3 only.

These error messages will hopefully help avoid a class of questions that are frequently asked on Stack Overflow. In particular, there was no error about incorrect types that made it to force_type, so users trying to return JSON with something like return True would get a mysterious "bool object is not callable" error.