Emit DNSimple API error details

[cttttt]

What's the problem?

When the DNSimple API returns validation errors, although the type of the API error is included in exceptions (Validation failed), the details of the error are omitted.

For example, if we somehow attempted to put a zone into an inconsistent state (and this passed through the validations in descendants of RecordStore::Record) an exception with the following message would be emitted by record_store:

Dnsimple::RequestError: Validation failed

This doesn't give us much to go on as far as how to do these validations in RecordStore itself.

How does this fix it?

It appears as though the DNSimple API includes these details in the errors field of responses. This change simply ensures that these details are included in exceptions.

So, instead of the error above, this change would add a little bit more flavour:

Validation failed: A CNAME record exists for inconsistent-records.dns-scratch.me, cannot add another record

What do I need from a review

• Another set of eyes for style.
• Any situations where we'd break where error responses do not contain these extra fields. I added a test case, but feel free to suggest more testing here.

Follow up

And don't worry. I'll follow up by trying to get this change into upstream. If they accept it, we can upgrade the dependancy here, and remove the monkey-patch.