gopls/internal/lsp/cache: fail workspace load on context cancellation

If the context was cancelled early during initialization, it was
possible that all module load scopes get skipped, because ParseMod
returns the context error. As a result, the subsequent load would
succeed trivially, even though the context was cancelled.

After analyzing the reinitialization codepath, it seems very plausible
to encounter this race, and it explains the flakiness of
TestReinitializeRepeatedly -- I have found nothing else that would
explain the failure mode observed in logs, that the reload bypasses
initialization.

Fix this by returning when context errors are encountered.

Fixes golang/go#57780

Change-Id: I3fb971503f280131c59146bc586da45dd2ed1126
Reviewed-on: https://go-review.googlesource.com/c/tools/+/495058
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>

gopls/internal/lsp/cache/view.go

@@ -752,16 +752,18 @@ func (s *snapshot) loadWorkspace(ctx context.Context, firstAttempt bool) (loadEr
 			// errors.
 			fh, err := s.ReadFile(ctx, modURI)
 			if err != nil {
-				if ctx.Err() == nil {
-					addError(modURI, err)
+				if ctx.Err() != nil {
+					return ctx.Err()
 				}
+				addError(modURI, err)
 				continue
 			}
 			parsed, err := s.ParseMod(ctx, fh)
 			if err != nil {
-				if ctx.Err() == nil {
-					addError(modURI, err)
+				if ctx.Err() != nil {
+					return ctx.Err()
 				}
+				addError(modURI, err)
 				continue
 			}
 			if parsed.File == nil || parsed.File.Module == nil {