Improve error handling (#175)

* Fix undefined error

* Fix null error

Author: dnicolson

lib/switchbot-device.js

@@ -443,6 +443,9 @@ class SwitchbotDevice {
 
       this._connect()
         .then(() => {
+          if (!this._chars) {
+            return reject("No characteristics available.");
+          }
           return this._write(this._chars.write, req_buf);
         })
         .then(() => {

lib/switchbot.js

@@ -132,7 +132,9 @@ class Switchbot {
             for (let addr in peripherals) {
               device_list.push(peripherals[addr]);
             }
-            resolve(device_list);
+            if (device_list.length) {
+              resolve(device_list);
+            }
           };
 
           // Set a handler for the 'discover' event