fix: server upgrades should have more descriptive error messages (#11476)

during rolling upgrade, provide a more descriptive error
message and discourage rolling upgrade in such situations,
allowing users to take action.

additionally also rename `slashpath -> pathutil` to avoid
a slighly mis-pronounced usage of `path` package.

cmd/auth-handler.go

@@ -498,6 +498,7 @@ func setAuthHandler(h http.Handler) http.Handler {
 			// Validate Authorization header if its valid for JWT request.
 			if _, _, authErr := webRequestAuthenticate(r); authErr != nil {
 				w.WriteHeader(http.StatusUnauthorized)
+				w.Write([]byte(authErr.Error()))
 				return
 			}
 			h.ServeHTTP(w, r)