Clean up graphqlpeg::parse_error exception messages

[wravery]

PEGTL has a default control template class (normal) which raises parse_error exceptions using the type name of the required rule that was not matched. Some of the types in my GraphQL grammar were ambiguous and hard to map to the grammar rule that expected them.

I refactored the grammar to use a custom control template and I encapsulated all of the must or if_must rules which are the only valid match in those locations with *_content rules that can be unambiguously specialized in the control template class.