Merge error types in apollo-compiler

[goto-bus-stop]

apollo-compiler has several error enums, which for users ultimately only end up being accessed behind a DiagnosticList or DiagnosticData. We don't really use the different error enums for anything internally either, so it would be simpler to just have everything in one big enum.

This doesn't strictly need to happen at any particular point because it's not exposed, but it would make it easier to add errors in the future because we won't need to think about where to put them or put them in a place where they don't really belong.