debug: duplicate notifications about missing dlv-dap when debug test codelens is used

[hyangah]

Extension version: v0.24.1-rc.3
no dlv-dap installed

How to repro:

• make sure dlv-dap isn't installed in the system.
• enable dlv-dap from settings.json,

"go.delveConfig": {
        "debugAdapter": "dlv-dap",
 }

• click any 'debug test' codelens from a test go file.

What I expected to see:
Extension suggesting dlv-dap installation.

What I saw instead:
Extension triggered duplicate notification popups with no clear tool installation instruction.

In DEBUG CONSOLE, I saw error message like "Couldn't find dlv-dap at the Go tools path, /Users/hakim/go, /Users/hakim/go or ..." followed by the same "connect to server error:..." error messages.

[suzmue]

I think this is because we attempt to connect each time a message is sent to the server, which I think happens twice on startup. To fix this we can add some code to only attempt to connect once.

[gopherbot]

Change https://golang.org/cl/310517 mentions this issue: src/goDebugFactory.ts: send the first error when starting dlv-dap

[gopherbot]

Change https://golang.org/cl/311452 mentions this issue: [release] src/goDebugFactory.ts: send the first error when starting dlv-dap