"Couldn't find the Firefox executable" when launching in attach mode

I am getting the `Couldn't find the Firefox executable` when launching in attach mode. I am asking myself now if this is a wanted behaviour, since I wouldn't expect that the executable is needed for that.

## Background
I was following the instructions for launching in `attach` mode [in the README](https://github.com/firefox-devtools/vscode-firefox-debug#attach).
Accordingly, I have set up a `launch.json` that contained the recommended block:
``` json
{
"version": "0.2.0",
"configurations": [
{
"name": "Attach to localhost",
"type": "firefox",
"request": "attach",
"url": "http://127.0.0.1/"
}
]
}
```
My firefox had the necessary `about:config` changes and launched fine in debug mode with
``` bash
"C:\Program Files\Mozilla Firefox\firefox.exe" -start-debugger-server
```
When I launched the debug session in vscode with this configuration I got the `Couldn't find the Firefox executable` error.

## Cause
The `firefox.executable` config was set globally to a wrong location.

Two things that I think would have improved my situation
- the error message would point out that this is set in the global extension settings
- the error would not be thrown since (correct me if I am wrong) the executable is not called at all in `attach` mode

Start with the attach-mode instructions in the README and reproduce the configuration using a globally incorrect firefox.executable setting. Trace how the attach launch reads that setting and produces the error; done means attach mode no longer requires an unused executable, or the message clearly identifies the global extension setting.

Written by the indexing model from the issue text.