Improve messaging around yarn watch

[code-asher]

It's not clear whether VS Code is just failing or whether it's still building. I think it might make sense to have the error say that VS Code is still building (when that's the case) on the error page and on the terminal (instead of the ENOENT). Once VS Code has finished we can also output another message on the terminal explicitly saying so.

See #3091

[stale]

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