Have ssh plugin report on connection error

Before the connection would fail and log to trace without anything
particularly informative for the client (they just saw 'unexpected error'.
Provide a more informative behavior for the client.

confluent_server/confluent/plugins/shell/ssh.py

@@ -72,7 +72,6 @@ class SshShell(conapi.Console):
         while self.connected:
             pendingdata = self.shell.recv(8192)
             if pendingdata == '':
-                self.datacallback(conapi.ConsoleEvent.Disconnect)
                 return
             self.datacallback(pendingdata)
 
@@ -103,6 +102,13 @@ class SshShell(conapi.Console):
             self.password = ''
             self.datacallback('\r\nlogin as: ')
             return
+        except paramiko.ssh_exception.NoValidConnectionsError as e:
+            self.datacallback(str(e))
+            self.inputmode = 0
+            self.username = ''
+            self.password = ''
+            self.datacallback('\r\nlogin as: ')
+            return
         self.inputmode = 2
         self.connected = True
         self.shell = self.ssh.invoke_shell()