Check cached sources with upstream after the cache TTL (closes #69) (#350)

internal/handlers/image_errors_internal_test.go

@@ -428,7 +428,7 @@ type failingFetcher struct {
 
 // Fetch returns f's error.
 func (f failingFetcher) Fetch(
-	context.Context, imagerequest.Source,
+	context.Context, imagerequest.Source, string, string,
 ) (*httpfetcher.FetchResult, error) {
 	return nil, f.err
 }