Fix validation for ActionResult return types

[evanw555]

Validate the type parameter of ActionResult, not ActionResult itself. Also improve the build error message for cases where an Action method's return type is a custom typeref but the @Action parameter doesn't specify a returnTyperef attribute.

Context:

For an Action method that has a custom typeref return type (e.g. CustomLong) yet doesn't indicate in the @Action parameter that it has a typeref class (e.g. CustomLongRef.class), the error will show up as the following:

• If the return type is CustomLong: com.linkedin.restli.server.ResourceConfigException: @Action method 'doAction' on class 'com.example.impl.ExampleResource' has an invalid return type 'com.example.CustomLong'. Expected a DataTemplate or a primitive
• If the return type is ActionResult<CustomLong>: an unhandled reflection exception.

After this change, the error message will be this for both CustomLong and ActionResult<CustomLong>: com.linkedin.restli.server.ResourceConfigException: @Action method 'doAction' on class 'com.example.impl.ExampleResource' has an invalid return type 'com.example.CustomLong'. Expected a DataTemplate or a primitive, or expected returnTyperef to be specified in the @Action annotation.

A couple notes:

• I'm not sure if getLogicalReturnClass(method) was used instead of the returnClass parameter (which is more accurate because it also unwraps ActionResult) on purpose, but using returnClass would be accurate as it would make validation consistent between return types T and ActionResult<T>.
• I just added more text to the existing exception, but would it make sense to check if the custom coercer exists and to display a more specific error message?
• This is backward-incompatible because it removes ActionResult from the set of acceptable Action return types (it shouldn't be there if we're checking the logical return type).