x/tools/gopls: improve workspace error messages

[findleyr]

Workspace misconfiguration is still a source of confusion for gopls users. Particularly with the introduction of go.work files, when a workspace is misconfigured it is not always clear what action needs to be taken to resolve the problem.

This issue tracks improving our error message for invalid build configurations, and generally re-evaluating our workspace error messages.

[findleyr]

v0.9.2 for at least the major workspace error message

[gopherbot]

Change https://go.dev/cl/421499 mentions this issue: internal/lsp: update the broken workspace message to mention go.work