Update error messages

Author: jasonpaulos

android/src/main/java/com/rnappauth/RNAppAuthModule.java

@@ -171,7 +171,7 @@ public void register(
                         promise
                 );
             } catch (Exception e) {
-                promise.reject("Failed to refresh token", e.getMessage());
+                promise.reject("registration_failed", e.getMessage());
             }
         } else {
             final Uri issuerUri = Uri.parse(issuer);
@@ -182,7 +182,7 @@ public void onFetchConfigurationCompleted(
                                 @Nullable AuthorizationServiceConfiguration fetchedConfiguration,
                                 @Nullable AuthorizationException ex) {
                             if (ex != null) {
-                                promise.reject("Failed to fetch configuration", getErrorMessage(ex));
+                                promise.reject("service_configuration_fetch_error", getErrorMessage(ex));
                                 return;
                             }
 
@@ -466,7 +466,7 @@ public void onRegistrationRequestCompleted(@Nullable RegistrationResponse respon
                     WritableMap map = RegistrationResponseFactory.registrationResponseToMap(response);
                     promise.resolve(map);
                 } else {
-                    promise.reject("Failed to refresh token", getErrorMessage(ex));
+                    promise.reject("registration_failed", getErrorMessage(ex));
                 }
             }
         };

ios/RNAppAuth.m

@@ -62,7 +62,7 @@ - (dispatch_queue_t)methodQueue
         [OIDAuthorizationService discoverServiceConfigurationForIssuer:[NSURL URLWithString:issuer]
                                                             completion:^(OIDServiceConfiguration *_Nullable configuration, NSError *_Nullable error) {
                                                                 if (!configuration) {
-                                                                    reject(@"RNAppAuth Error", [error localizedDescription], error);
+                                                                    reject(@"service_configuration_fetch_error", [error localizedDescription], error);
                                                                     return;
                                                                 }
                                                                 [self registerWithConfiguration: configuration
@@ -241,7 +241,7 @@ - (void)registerWithConfiguration: (OIDServiceConfiguration *) configuration
                                                  if (response) {
                                                      resolve([self formatRegistrationResponse:response]);
                                                  } else {
-                                                     reject(@"RNAppAuth Error", [error localizedDescription], error);
+                                                     reject(@"registration_failed", [error localizedDescription], error);
                                                  }
                                             }];
 }