Tune a bit an error message

src/server/kiwix-serve.cpp

@@ -84,7 +84,7 @@ string loadCustomTemplate (string customIndexPath) {
                       kiwix::computeAbsolutePath(kiwix::getCurrentDirectory(), customIndexPath) :
                       customIndexPath;
   if (!kiwix::fileExists(customIndexPath)) {
-    throw runtime_error("No such file exist " + customIndexPath);
+    throw runtime_error("No such file exist (or file is not readable)" + customIndexPath);
   }
   if (kiwix::getMimeTypeForFile(customIndexPath) != "text/html") {
     throw runtime_error("Invalid File Mime Type " + kiwix::getMimeTypeForFile(customIndexPath));