js: change db error to match firebase web sdk error

lib/modules/database/reference.js

@@ -77,12 +77,12 @@ export default class Reference extends ReferenceBase {
    */
   push(value: any, onComplete: Function) {
     if (value === null || value === undefined) {
-      const _path = this.path + '/' + generatePushID(this.db.serverTimeOffset);
-      return new Reference(this.db, _path);
+      return new Reference(this.db, `${this.path}/${generatePushID(this.db.serverTimeOffset)}`);
     }
 
     const path = this._dbPath();
     const _value = this._serializeAnyType(value);
+
     return promisify('push', FirebaseDatabase)(path, _value)
       .then(({ ref }) => {
         const newRef = new Reference(this.db, ref);
@@ -102,7 +102,7 @@ export default class Reference extends ReferenceBase {
    * @param context TODO
    * @returns {*}
    */
-  on(eventType: string, successCallback: () => any, failureCallback: () => any, context) {
+  on(eventType: string, successCallback: () => any, failureCallback: () => any) {
     if (!isFunction(successCallback)) throw new Error('The specified callback must be a function');
     if (failureCallback && !isFunction(failureCallback)) throw new Error('The specified error callback must be a function');
     const path = this._dbPath();
@@ -280,14 +280,27 @@ export default class Reference extends ReferenceBase {
     return newRef;
   }
 
+  /**
+   *
+   * @returns {Disconnect}
+   */
   onDisconnect() {
     return new Disconnect(this.path);
   }
 
+  /**
+   * Get a specified child
+   * @param path
+   * @returns {Reference}
+   */
   child(path: string) {
-    return new Reference(this.db, this.path + '/' + path);
+    return new Reference(this.db, `${this.path}/${path}`);
   }
 
+  /**
+   * Return the ref as a path string
+   * @returns {string}
+   */
   toString(): string {
     return this._dbPath();
   }