Reworked error handling

[milessabin]

• Result is now a first class type which can represent a Success (a result of type T), a Warning (a result and also client-reportable GraphQL errors), a Failure (client-reportable GraphQL errors only) and an InternalError (an error which should not be reported to clients). Result has instances for MonadError, Traverse, Eq and Semigroup and is accompanied by a ResultT transformer.

  This is a breaking change, but its main manifestation in user code is that elaborators and effects need to use Result's .success syntax instead of Ior's .rightIor syntax.

• Mapping's effect now requires a MonadThrow instance rather than a Monad. This is to allow internal errors in a final Result to be unpacked into the effect. This allows us to return an F[Json] or a Stream[F, Json] rather than an F[Result[Json]] or Stream[F, Result[Json]] which improves the user ergonomics somewhat.

  Unfortunately the use of Streams means that for the pure tests in core we can't replace Id with Either[Throwable, *] because there is no instance of fs2.Compiler for that type. Rather than jump through hoops to support pure operation, which is only likely to be useful in tests, the formerly pure tests have simply been switched over to IO.

• Several cases where illegal states were silently ignored are now correctly reported as internal errors.

• All failure cases have been checked to ensure that internal errors will not be leaked to clients.

• All occurrences of sys.error have been eliminated.

• All assertions now represent logic errors in Grackle. Whilst it's possible that these might be triggered by (for example) user errors in mappings, the implication is that these should have been caught and reported prior to hitting an assertion.

Fixes #199 and #200.

[tpolecat]

👍 This is great. SqlMapping needed a lot of pushing-about but it's much improved I think. If it's not too much of as hassle I would like to see law-checking for Result, but feel free to merge and do it as a follow-up if you prefer.