Fix error-handling for unknown type in asObjectiveCNumberType. (#1152)

We were not declaring "error", so ended up throwing an "undeclared variable"
exception, not the actual exception we were trying to throw.

src-electron/generator/matter/darwin/Framework/CHIP/templates/helper.js

@@ -110,14 +110,15 @@ function asObjectiveCNumberType(label, type, asLowerCased) {
             return 'Float';
           case 'double':
             return 'Double';
-          default:
-            error =
+          default: {
+            let error =
               label +
               ': Unhandled underlying type ' +
               zclType +
               ' for original type ' +
               type;
             throw error;
+          }
         }
       })
       .then((typeName) =>