Inform a user about port binding failure

[m1kola]

First of all thank you for the extension - I recently discovered it and it works very well.

I have a suggestion about how to make the extension to be more user-friendly.

9005 is currently a default port for the extension and I had something listening on it when I first tried the extension. I saw a blank screen instead of a preview tab and was a bit puzzled by it, because currently when the extension fails to bind to a port it fails silently and leaves a user without a clue:

vs-openapi-designer/src/extension.js

Line 20 in 81722de

server = oadServer(port, openApiFile);

I would add an error message explaining what happened and suggesting to change the port using the openApiDesigner.defaultPort setting.