Invalid Argument errors provide per field details. (#110)

* Invalid Argument errors now return a per field error message.

* Creating an API error struct.

* SDK uses field error details proto.

* Adding tests specifically for handler helper error messages resulting in expected API output.

* Inlining the error checks into the table driven tests.  Use protocmp for assertions to make printing out diffs easier to parse.

* Aggregate the invalid argument fields detected to be wrong.

* Make the returned order of fields deterministic in the error details..

* Fixing formatting issue in errors_test.go.

* Test to confirm unimplemented results in a 405 and not 404.

internal/proto/local/controller/api/v1/error.proto

@@ -7,7 +7,13 @@ option go_package = "github.com/hashicorp/watchtower/internal/gen/controller/api
 message ErrorDetails {
 	string TraceId = 1 [json_name="TraceId"];
 	string request_id = 2 [json_name="request_id"];
-	repeated string request_fields = 3 [json_name="request_fields"];
+	repeated FieldError request_fields = 3 [json_name="request_fields"];
+}
+
+// FieldErrors contains error information on a per field basis.
+message FieldError {
+	string name = 1;
+	string description= 2;
 }
 
 // Error is returned by the JSON API when an error occurs.