Improve dmsgpty-ui logging when visor is no dmsgpty instance.

[Senyoret1]

Describe the bug
When opening the dmsgpty-ui terminal for some visors, it works well, but for some others it just stays black.

Environment information:

• OS: Linux (Ubuntu 18.04.1)
• Platform: Linux 4.15.0-65-generic x86_64

Steps to Reproduce
Steps to reproduce the behavior:

1. Start some nodes using the make integration-run-generic command of the skywire-services repository (tested using the milestone2 branch of the skywire-mainnet repo and the mainnet-milestone2 branch of the skywire-services repo).

2. Open in a browser https://{{ host }}/pty/024ec47420176680816e0406250e7156465e4531f5b26057c9f6297bb0303558c7 to open the the dmsgpty-ui terminal for VisorA. The terminal should work well on the browser and the command window for VisorA will show:
   

3. Open in a browser https://{{ host }}/pty/0348c941c5015a05c455ff238af2e57fb8f914c399aab604e9abb5b32b91a4c1fe to open the the dmsgpty-ui terminal for VisorB. The terminal will stay black and the command window for VisorB will show:
   

Actual behavior
The terminal for VisorB does not work.

Expected behavior
The terminal should work.

Additional context

Possible implementation

[evanlinjin]

It means the visor has no dmsgpty instance associated with it. This is expected behaviour, although the logs can be clearer.

[Senyoret1]

Good 👍. I reported this because I added a buton to the manager for opening the dmsgpty terminal on a new tab and had no way to know why the terminal showed nothing. In terms of functionality there is no big problem because the manager also has a simple terminal which works with the exec API endpoint, so it is possible to continue working if the visor does not have a dmsgpty instance associated with it.

In terms of the UX, it would be good to inform the user in the UI what happened, not just on the terminal were the visor is running. A way to do this could simply be to return an error text in the html obtained from the /pty URL, instead of the code for showing the terminal, which will not work, because the visor does not have a dmsgpty instance associated with it.

[jdknives]

https://github.com/SkycoinProject/dmsg/pull/88 Closed by this PR.