internal/frontend: allow refetching of 500s

If a fetch request previously resulted in a 500, users will now be able
to re-request for that path. Previously, they would be presented with a
500.

Change-Id: I8fa4dcf7f0f90e1b234e29e5bf4dbf8375ee7eb5
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/259317
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/internal/frontend/details.go b/internal/frontend/details.go
index c70269b..ea1c19f 100644
--- a/internal/frontend/details.go
+++ b/internal/frontend/details.go
@@ -421,7 +421,7 @@
 	}
 	results := s.checkPossibleModulePaths(ctx, db, fullPath, requestedVersion, modulePaths, false)
 	for _, fr := range results {
-		if fr.status == statusNotFoundInVersionMap {
+		if fr.status == statusNotFoundInVersionMap || fr.status == http.StatusInternalServerError {
 			// If the result is statusNotFoundInVersionMap, it means that
 			// we haven't attempted to fetch this path before. Return an
 			// error page giving the user the option to fetch the path.