Raise a SchemaError when a RecordSchema is missing a fields array

It used to raise a NoMethodError with the following message "undefined method `each_with_index' for nil:NilClass"
when it was presented with a SchemaRecord that didn't include a fields array.

This PR makes the SchemaRecord raise a readable SchemaParseError instead. I couldn't find any specs regarding schema parsing and parse errors so I could only test this manually.