internal/lsp/source: include setting name in all settings errors
internal/lsp/source: include setting name in all settings errors

Every setting parse error should include the name of the error for user
sanity. Add it in the errorf method, clean up redundancy, and make
existing errors more consistent/correct.

Updates golang/go#41964.

Change-Id: I8f980ceb5d82d4238edcd4095ea0e9572ac9c271
Reviewed-on: https://go-review.googlesource.com/c/tools/+/262353
Trust: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools