x/tools/gopls: improve the `gopls bug` subcommand, and use it more widely

[findleyr]

Following https://go.dev/issue/60963, we had a discussion around when it is correct to use log.Fatal, panic, or bug.Reportf, when encountering an "impossible" condition in gopls.

Summarizing conclusions:

• If it is at all possible to proceed, use bug.Reportf or bug.Errorf and keep going. (the bug package was written to allow making gopls incrementally more assertive, without crashing).
• If it is impossible to proceed, and the condition is provably impossible, use log.Fatal
• If it is impossible to proceed, and we can't prove that the condition is impossible, use panic (this will allow VS Code crash reporting to capture the stack, among other things).

Action items:

• populate the issue template created by the gopls bug subcommand with any bug reports captured in the file cache
• don't just write the first bug report to the file cache: write one exemplar per callsite, and (maybe) track a count per callsite
• add a section on error handling to a contributor guide inside the x/tools/gopls/doc directory.

[gopherbot]

Change https://go.dev/cl/505579 mentions this issue: gopls/internal/lsp/cmd: list bug reports in 'gopls bug'

[gopherbot]

Change https://go.dev/cl/506938 mentions this issue: gopls/doc/contributing.md: document error handling strategies