Add a warning about bad install

Thanks to https://github.com/react-native-community/react-native-netinfo/blob/v4.2.1/src/internal/nativeInterface.ts#L15

src/index.ts

 }
 
 const RNPermissions: {
-  openSettings: () => Promise<true>;
-  check: (permission: Permission) => Promise<PermissionStatus>;
-  request: (
-    permission: Permission,
-    rationale?: Rationale,
-  ) => Promise<PermissionStatus>;
+  // Android + iOS
   checkNotifications: () => Promise<NotificationsResponse>;
   requestNotifications: (
     options: NotificationOption[],
   ) => Promise<NotificationsResponse>;
+  openSettings: () => Promise<true>;
 
-  // android only module methods
+  // Android only
   isAvailable: (permission: Permission) => Promise<boolean>;
-  setNonRequestable: (permission: Permission) => Promise<true>;
   getNonRequestables: () => Promise<Permission[]>;
+  setNonRequestable: (permission: Permission) => Promise<true>;
+
+  // iOS only
+  check: (permission: Permission) => Promise<PermissionStatus>;
+  request: (
+    permission: Permission,
+    rationale?: Rationale,
+  ) => Promise<PermissionStatus>;
 } = NativeModules.RNPermissions;
 
+// Produce an error if we don't have the native module
+if (RNPermissions == null) {
+  throw new Error(`react-native-permissions: NativeModule.RNPermissions is null. To fix this issue try these steps:
+• Run \`react-native link react-native-permissions\` in the project root.
+• Rebuild and re-run the app.
+• If you are using CocoaPods on iOS, run \`pod install\` in the \`ios\` directory and then rebuild and re-run the app. You may also need to re-open Xcode to get the new pods.
+• Check that the library was linked correctly when you used the link command by running through the manual installation instructions in the README.
+* If you are getting this error while unit testing you need to mock the native module. Follow the guide in the README.
+If none of these fix the issue, please open an issue on the Github repository: https://github.com/react-native-community/react-native-permissions`);
+}
+
 const platformPermissions = Object.values(
   Platform.OS === 'ios' ? IOS : ANDROID,
 );