Add handling of login failure in examples

Check and handle login failure in the examples provided for using this
library.  This is a common error condition that should be handled by any
script using this library.

examples/deleted.py

@@ -3,6 +3,10 @@ import discord
 client = discord.Client()
 client.login('email', 'password')
 
+if not client.is_logged_in:
+    print('Logging in to Discord failed')
+    exit(1)
+
 @client.event
 def on_ready():
     print('Connected!')

examples/echo.py

@@ -3,6 +3,10 @@ import discord
 client = discord.Client()
 client.login('email', 'password')
 
+if not client.is_logged_in:
+    print('Logging in to Discord failed')
+    exit(1)
+
 @client.event
 def on_ready():
     print('Connected!')

examples/edits.py

@@ -4,6 +4,10 @@ import time
 client = discord.Client()
 client.login('email', 'password')
 
+if not client.is_logged_in:
+    print('Logging in to Discord failed')
+    exit(1)
+
 @client.event
 def on_ready():
     print('Connected!')

examples/reply.py

@@ -3,6 +3,10 @@ import discord
 client = discord.Client()
 client.login('email', 'password')
 
+if not client.is_logged_in:
+    print('Logging in to Discord failed')
+    exit(1)
+
 @client.event
 def on_message(message):
     if message.content.startswith('!hello'):