internal/lsp: return errors when we can't find a package

This will allow us to surface these errors as diagnostics.

Change-Id: Ic8ad11ba43c08be6f391c0e2aa95015183461133
Reviewed-on: https://go-review.googlesource.com/c/tools/+/173780
Reviewed-by: Ian Cottrell <iancottrell@google.com>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>

internal/lsp/cache/check.go

@@ -91,7 +91,13 @@ func (v *View) checkMetadata(ctx context.Context, f *File) ([]packages.Error, er
 			if err == nil {
 				err = fmt.Errorf("no packages found for %s", f.filename)
 			}
-			return nil, err
+			// Return this error as a diagnostic to the user.
+			return []packages.Error{
+				{
+					Msg:  err.Error(),
+					Kind: packages.ListError,
+				},
+			}, err
 		}
 		for _, pkg := range pkgs {
 			// If the package comes back with errors from `go list`, don't bother

internal/lsp/cache/file.go

@@ -89,8 +89,7 @@ func (f *File) GetPackage(ctx context.Context) source.Package {
 	f.view.mu.Lock()
 	defer f.view.mu.Unlock()
 	if f.pkg == nil || len(f.view.contentChanges) > 0 {
-		errs, err := f.view.parse(ctx, f)
-		if err != nil {
+		if errs, err := f.view.parse(ctx, f); err != nil {
 			// Create diagnostics for errors if we are able to.
 			if len(errs) > 0 {
 				return &Package{errors: errs}