fix go lint warnings

- handle errors where possible
- explicitly ignore errors where it makes sense to
- fix deprecations and unused var

internal/s3cache/s3cache.go

@@ -75,7 +75,7 @@ func (c *cache) Delete(key string) {
 
 func keyToFilename(key string) string {
 	h := md5.New()
-	io.WriteString(h, key)
+	_, _ = io.WriteString(h, key)
 	return hex.EncodeToString(h.Sum(nil))
 }