Picker item polish

[joyceerhl]

Testing microsoft/vscode-jupyter#14367

1. Enter the URL of the running JupyterHub Server... could be shortened to Enter URL of running JupyterHub server in the quickpick item name and placeholder
2. Invalid JupyterHub URL is pretty vague and hard to troubleshoot (could be because there isn't a server at that URL, or because of some other connectivity error), would be nice to either include the actual error message or provide a command link to open the extension output channel for logs
   
3. If the username and password are wrong we show the username input again and say Invalid username or password., there shouldn't be a period at the end of that as it's inconsistent with the other validation messages that we show, and ideally it should say something more actionable like Your username or password did not match (even better if we can say which part was incorrect)

[DonJayamanne]

even better if we can say which part was incorrect)

As a best practice in security this is never done, else anyone trying to hack your account know they have made progress and need to only focus on the other

error message or provide a command link to open the extension output channel for logs

To my knowledge cors errors cannot be detected,

[DonJayamanne]

could be shortened to Enter URL of running JupyterHub server in the quickpick item name and placeholder

Leaving this as is, as this is what we have in the Jupyter extension for standard urls.