fix: Handle server disconnects / non-existing server

[bmingles]

This PR improves the server connection experience / fixes some bugs:

• Status bar item now shows Deephaven: Connecting... while a connection is being initialized (before it immediately showed as connected)
• Status bar shows Deephaven: Disconnected if connection fails
• If client disconnects due to server stopping or anything else that results in an Jsapi ide 'disconnect' event, the status bar will be updated to reflect the status
• Updated status bar / quick pick menu icons

Testing

Setup
If you clone this repo and run npm install, you should be able to f5 to run the extension in debug mode.

Test Steps

• Start a DH server on localhost:10000
• Connect to server in extension. Should see status bar "Connecting..." then "Connected" status (the connecting status may happen too quickly to really see it).
• Kill server
• Should see status bar item switch to disconnected
• Attempt to re-connect to stopped server
• Should see "Connecting..." status for longer this time and then a "Failed to initialize Deephaven API" toast. Status should say "Disconnected"

fixes #4 fixes #5 fixes #6