internal/lsp/cache: keep going on per-file workspace symbol errors
internal/lsp/cache: keep going on per-file workspace symbol errors

I'm not sure how to hit this in practice, since it appears that symbols
should only return errors with some type of corruption, but we should
keep going when searching for symbols in snapshot files.

Change-Id: I9fcf6cc92fdc943520e8890ece51d28ec46e8812
Reviewed-on: https://go-review.googlesource.com/c/tools/+/376363
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Peter Weinberger <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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