Fix tests, add PanicOnError option

- Helper method for internal server errors with consistent logging.
- Add PanicOnError option to panic on internal server errors. This
  makes it easier to traces where the condition was hit in testing.

handlers.go

@@ -27,6 +27,7 @@ type Server struct {
 	Prometheus   bool
 	Debug        bool
 	MaxRepoSize  int64
+	PanicOnError bool
 
 	htpasswdFile *HtpasswdFile
 	quotaManager *quota.Manager
@@ -85,6 +86,7 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		AppendOnly:   s.AppendOnly,
 		Debug:        s.Debug,
 		QuotaManager: s.quotaManager, // may be nil
+		PanicOnError: s.PanicOnError,
 	}
 	if s.Prometheus {
 		opt.BlobMetricFunc = makeBlobMetricFunc(username, folderPath)