validate method return types

[vektah]

Wow, this was bigger than I thought it would be.

Fixes a few type validation bugs when binding, and now returns proper error messages if binding fails for input types (its not possible to fall back to a resolver for inputs)

eg:

Unable to bind RecursiveInputSlice.self to github.com/vektah/gqlgen/codegen/testdata.RecursiveInputSlice
  no method named self
  field has wrong type: []github.com/vektah/gqlgen/codegen/testdata.RecursiveInputSlice is not compatible with *[]*github.com/vektah/gqlgen/codegen/testdata.RecursiveInputSlice
exec plan failed

These error messages can be shown for object types too, but they aren't errors because resolvers will be generated. If you run with gqlgen -v you will see why resolvers are being generated
eg:

example/scalars$ gqlgen -v
Unable to bind User.primitiveResolver to github.com/vektah/gqlgen/example/scalars/model.User
  no method named primitiveResolver
  no field named primitiveResolver
  Adding resolver method
Unable to bind User.customResolver to github.com/vektah/gqlgen/example/scalars/model.User
  no method named customResolver
  no field named customResolver
  Adding resolver method
example/scalars$ echo $?
0

Without the -v there will be no output.

fixes #162