Move validation errors into a pkg

This is a precursor to fixing my other PR about error lists - That one is on hold until this or something like it goes in.

This makes ErrorList a public type, but does not implement the error interface on it (because I think we want an empty list to not be an error).  ErrorList.ToError() produces an error or nil if the list is empty.  I put it under api/errors, because that is what it is used for, and the Make* functions are sort of validation-centric.
Do we need to prefix all of these functions with "Make"? Callers will be prefixing them with the package name, and I think errors.InvalidError(...) reads just as well and is more concise than errors.MakeInvalidError(...).