internal/lsp/cache: add missing error checks
internal/lsp/cache: add missing error checks

I wrote this code as if there was going to be a final error check after
all the type checking attempts, but ended up using the result inside the
attempts, errors would likely have resulted in panics. Just do normal,
non-clever error checking.

Change-Id: I665f34f7e6d1a2c3465543cbdc39a723a22a1095
Reviewed-on: https://go-review.googlesource.com/c/tools/+/319371
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