internal/frontend: don't fail if module lacks readme

When extracting links, don't return an error if the module
readme isn't present.

Change-Id: I0be8853e630c727c86d2e0adb6bf3f261b558495
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/276294
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
diff --git a/internal/frontend/unit_main.go b/internal/frontend/unit_main.go
index 42468b4..6b191e9 100644
--- a/internal/frontend/unit_main.go
+++ b/internal/frontend/unit_main.go
@@ -167,14 +167,16 @@
 	// not, we will not show the module links on the unit page.
 	if unit.Path != unit.ModulePath && unit.IsRedistributable && experiment.IsActive(ctx, internal.ExperimentGoldmark) {
 		modReadme, err := ds.GetModuleReadme(ctx, unit.ModulePath, unit.Version)
-		if err != nil {
+		if err != nil && !errors.Is(err, derrors.NotFound) {
 			return nil, err
 		}
-		rm, err := processReadme(modReadme, um.SourceInfo)
-		if err != nil {
-			return nil, err
+		if err == nil {
+			rm, err := processReadme(modReadme, um.SourceInfo)
+			if err != nil {
+				return nil, err
+			}
+			modLinks = rm.Links
 		}
-		modLinks = rm.Links
 	}
 
 	return &MainDetails{