x/tools/gopls: unknown revision error doesn't propagate to diagnostics

[stamblerre]

In conversation with @ardan-bkennedy, we noticed that adding a require github.com/pkg/errors v0.8.3 caused a diagnostic to get stuck because it was an unknown revision. Error messages should either get propagated, or we should clear diagnostics so they don't appear to be stuck?

[gopherbot]

Change https://golang.org/cl/242159 mentions this issue: internal/lsp: handle unknown revision in go.mod file

[gopherbot]

Change https://golang.org/cl/242477 mentions this issue: internal/lsp: try to parse diagnostics out of go list errors