Cordova Contacts plugin fix

In find() method the onError callback should be optional.

https://cordova.apache.org/docs/en/3.3.0/cordova_contacts_contacts.md.ht
ml#contacts.find

cordova/plugins/Contacts.d.ts

@@ -32,7 +32,7 @@ interface Contacts {
      */
     find(fields: string[],
         onSuccess: (contacts: Contact[]) => void,
-        onError: (error: ContactError) => void,
+        onError?: (error: ContactError) => void,
         options?: ContactFindOptions): void;
 }