Made required_draft4 more robust when used with Draft3

[wspeirs]

When I try to validate a Draft3 schema using the Draft4 validator, you get the following error:

  File "/usr/lib/python3/dist-packages/jsonschema/_validators.py", line 299, in required_draft4
    for property in required:
TypeError: 'bool' object is not iterable

My change fixes this issue by alerting the use to the mis-match in draft versions.

[Julian]

Hi! Thanks.

So, this would only happen if you didn't first verify you had a valid schema by using Draft4Validator.check_schema before you called Draft4Validator(schema).validate -- if you did, you'd see that the meta schema complains about having the wrong type there (which is why `jsonschema.validate calls that first before calling the method on the validator).

Generally, all the validators ask for and document that they need to be given valid schemas for their errors to be decipherable.

[wspeirs]

I was presented this error when using the command line tool. So maybe the real fix is to change that to call `check_schema` first?

[Julian]

Yes! Definitely, that is issue #208 -- would definitely appreciate a patch for that.

[wspeirs]

@Julian updated my PR to include a fix for #208 as well. If you want, you can cherry pick b44c1a2 but I think it's safe to pull in both changes.

[Julian]

Seems like it's in the right place, can you add a test?

[wspeirs]

@Julian added a unit test in 788ea8e

[Julian]

Merged (at least the part that's correct here). Thanks -- really appreciate it.