Better error handling

[candlerb]

I am using SSHy with Chrome 85.

When something goes wrong, error handling isn't great. In some cases, all I see is the "spinner" spinning (or in the case of wrapper.html, just a black screen), and no other error displayed.

Here are some ways to replicate this:

• Simply don't start wsproxy. tcpdump on port 5999 shows a SYN and RST exchange. SSHy shows nothing, and the spinner just keeps going.
• Run the stock version of wsproxy, instead of the stuicey one. tcpdump shows the exchange getting as far as SSH-2.0-OpenSSH_7.6p1 Ubuntu-4ubuntu0.3, at which point it hangs with the spinner spinning.

Another minor bug:

• Connect with a wrong password. SSHy correctly says "Invalid Username or Password" in the dialog box. But if you then enter the correct password and hit enter again, the terminal displays WebSocket connection failed: Error in connection establishment: code 1005. You have to refresh the screen and enter the target, username and (correct) password again.