#52 handle failed hosts better

Made ```join``` and ```get_exit_codes``` handle failed hosts gracefully when ```stop_on_errors=False``` is passed to run_command

Author: iameugenejo

pssh/pssh_client.py

@@ -401,8 +401,10 @@ def join(self, output):
         """Block until all remote commands in output have finished
         and retrieve exit codes"""
         for host in output:
-            for line in output[host]['stdout']:
-                pass
+            stdout = output[host].get('stdout')
+            if stdout:
+                for _ in stdout:
+                    pass
         self.get_exit_codes(output)
 
     def get_exit_codes(self, output):
@@ -429,7 +431,7 @@ def get_exit_code(self, host_output):
 
     def _get_exit_code(self, channel):
         """Get exit code from channel if ready"""
-        if not channel.exit_status_ready():
+        if not channel or not channel.exit_status_ready():
             return
         channel.close()
         return channel.recv_exit_status()