Improve the error message on saving when offline (fixes #2373)

data/core.yaml

@@ -246,6 +246,7 @@ en:
     help: "Save changes to OpenStreetMap, making them visible to other users."
     no_changes: No changes to save.
     error: An error occurred while trying to save
+    unknown_error_details: "Please ensure you are connected to the internet."
     uploading: Uploading changes to OpenStreetMap.
     unsaved_changes: You have unsaved changes
   success:

dist/locales/en.json

@@ -301,6 +301,7 @@
         "help": "Save changes to OpenStreetMap, making them visible to other users.",
         "no_changes": "No changes to save.",
         "error": "An error occurred while trying to save",
+        "unknown_error_details": "Please ensure you are connected to the internet.",
         "uploading": "Uploading changes to OpenStreetMap.",
         "unsaved_changes": "You have unsaved changes"
     },

js/id/modes/save.js

@@ -30,7 +30,7 @@ iD.modes.Save = function(context) {
                     confirm
                         .select('.modal-section.message-text')
                         .append('p')
-                        .text(err.responseText);
+                        .text(err.responseText || t('save.unknown_error_details'));
                 } else {
                     context.flush();
                     success(e, changeset_id);