service_authorization: Use dedicated errors

Instead of re-using amibuous error messages, create and use a dedicated
error when CreateServiceAuthorization is used incorrectly.

fastly/errors.go

@@ -171,6 +171,14 @@ var ErrMissingServerSideEncryptionKMSKeyID = NewFieldError("ServerSideEncryption
 // requires a "ServiceID" key, but one was not set.
 var ErrMissingServiceID = NewFieldError("ServiceID")
 
+// ErrMissingServiceAuthorizationsService is an error that is returned when an input struct
+// requires a "ServiceAuthorizationService" key, but one was not set.
+var ErrMissingServiceAuthorizationsService = NewFieldError("Service").Message("SAService requires an ID")
+
+// ErrMissingServiceAuthorizationsUser is an error that is returned when an input struct
+// requires a "ServiceAuthorizationUser" key, but one was not set.
+var ErrMissingServiceAuthorizationsUser = NewFieldError("User").Message("SAUser requires an ID")
+
 // ErrMissingUserID is an error that is returned when an input struct
 // requires a "UserID" key, but one was not set
 var ErrMissingUserID = NewFieldError("UserID")

fastly/service_authorization.go

@@ -69,10 +69,10 @@ type CreateServiceAuthorizationInput struct {
 // CreateServiceAuthorization creates a new service authorization granting granular service and user permissions.
 func (c *Client) CreateServiceAuthorization(i *CreateServiceAuthorizationInput) (*ServiceAuthorization, error) {
 	if i.Service == nil || i.Service.ID == "" {
-		return nil, ErrMissingServiceID
+		return nil, ErrMissingServiceAuthorizationsService
 	}
 	if i.User == nil || i.User.ID == "" {
-		return nil, ErrMissingUserID
+		return nil, ErrMissingServiceAuthorizationsUser
 	}
 
 	resp, err := c.PostJSONAPI("/service-authorizations", i, nil)

fastly/service_authorization_test.go

@@ -106,15 +106,15 @@ func TestClient_CreateServiceAuthorization_validation(t *testing.T) {
 		Service: &SAService{ID: ""},
 		User:    &SAUser{ID: ""},
 	})
-	if err != ErrMissingServiceID {
+	if err != ErrMissingServiceAuthorizationsService {
 		t.Errorf("bad error: %s", err)
 	}
 
 	_, err = testClient.CreateServiceAuthorization(&CreateServiceAuthorizationInput{
 		Service: &SAService{ID: "my-service-id"},
 		User:    &SAUser{ID: ""},
 	})
-	if err != ErrMissingUserID {
+	if err != ErrMissingServiceAuthorizationsUser {
 		t.Errorf("bad error: %s", err)
 	}
 }