Retry loading map on variable error

If the back is getting an error (because the user has no right to set a variable),
instead of failing directly, let's try to reload the map (maybe we have cached a wrong version of the map).

back/src/Services/VariableError.ts

@@ -0,0 +1,9 @@
+/**
+ * Errors related to variable handling.
+ */
+export class VariableError extends Error {
+    constructor(message: string) {
+        super(message);
+        Object.setPrototypeOf(this, VariableError.prototype);
+    }
+}