Improve error messages when issues are detected with formats

[Ben Sayers]

When a mock value fails to validate due to not being compatible with formats in a swagger file the following error is shown to the user:

'Response body is incompatible with the response body schema in the swagger file: should pass "formatDouble" keyword validation'

It is difficult to understand what is wrong with the mock value based on this error. This enhancement is to improve these errors. The errors are generated by ajv when our custom keywords fail validation.

Related: https://bitbucket.org/atlassian/swagger-mock-validator/issues/67/double-significant-digits