tests: Teach `waitFor()` to reject if the predicate throws

tests/frontend/helper.js

@@ -169,16 +169,14 @@ var helper = {};
     };
 
     var intervalCheck = setInterval(function(){
-      var passed = false;
-
-      passed = conditionFunc();
-
-      if(passed){
+      try {
+        if (!conditionFunc()) return;
+        deferred.resolve();
+      } catch (err) {
+        deferred.reject(err);
+      }
       clearInterval(intervalCheck);
       clearTimeout(timeout);
-
-        deferred.resolve();
-      }
     }, intervalTime);
 
     var timeout = setTimeout(function(){

tests/frontend/specs/helper.js

@@ -155,6 +155,15 @@ describe("the test helper", function(){
       });
     });
 
+    it('rejects if the predicate throws', async function() {
+      let err;
+      await helper.waitFor(() => { throw new Error('test exception'); })
+          .fail(() => {}) // Suppress the redundant uncatchable exception.
+          .catch((e) => { err = e; });
+      expect(err).to.be.an(Error);
+      expect(err.message).to.be('test exception');
+    });
+
     describe("returns a deferred object", function(){
       it("it calls done after success", function(done){
         helper.waitFor(function(){