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.

api/error_details.gen.go

@@ -12,9 +12,9 @@ import (
 type ErrorDetails struct {
 	defaultFields []string
 
-	TraceId       *string  `json:"TraceId,omitempty"`
-	RequestId     *string  `json:"request_id,omitempty"`
-	RequestFields []string `json:"request_fields,omitempty"`
+	TraceId       *string       `json:"TraceId,omitempty"`
+	RequestId     *string       `json:"request_id,omitempty"`
+	RequestFields []*FieldError `json:"request_fields,omitempty"`
 }
 
 func (s *ErrorDetails) SetDefault(key string) {