fix(mocks): $timeout#flush should not update time when empty

When $timeout#flush is called with a delay and no task can be flushed within that
delay, the current time should not be updated as that gets the mock into an inconsistent
state.

BREAKING CHANGE: if a tests was written around the buggy behavior the delays might be off now

This would typically not be a problem, but because of the previous breaking change in
$timeout.flush, the combination of two might be confusing and that's why we are documenting
it.

Old behavior:

```
doSomething(); //schedules task to execute in 500ms from now
doOtherStuff(); //schedules task to execute in 600ms from now

try {
  $timeout.flush(300); // throws "no task to be flushed" exception
} catch(e) {};
$time.flush(200); //flushes only doSomething() task
```

New behavior:

```
doSomething(); //schedules task to execute in 500ms from now
doOtherStuff(); //schedules task to execute in 600ms from now

try {
  $timeout.flush(300); // throws "no task to be flushed" exception
} catch(e) {};
$time.flush(200); // throws "no task to be flushed" exception again
                  // because previous exception didn't move the time forward
```

Fixed test:

```
doSomething(); //schedules task to execute in 500ms from now
doOtherStuff(); //schedules task to execute in 600ms from now

try {
  $timeout.flush(300); // throws "no task to be flushed" exception
} catch(e) {};
$time.flush(500); // flushes only doSomething() task
```

test/ngMock/angular-mocksSpec.js

@@ -404,7 +404,7 @@ describe('ngMock', function() {
       expect(function() {$timeout.flush(100);}).toThrow();
       expect(log).toEqual(['t1']);
 
-      $timeout.flush(900);
+      $timeout.flush(1000);
       expect(log).toEqual(['t1', 't2']);
       expect(function() {$timeout.flush();}).toThrow();
     });
@@ -425,6 +425,21 @@ describe('ngMock', function() {
     });
 
 
+    it('should not update the current time if an exception is thrown during a flush', function() {
+      $timeout(log.fn('t1'), 100);
+      $timeout(log.fn('t2'), 101);
+
+      expect(function() { $timeout.flush(90); }).toThrow();
+      expect(function() { $timeout.flush(90); }).toThrow();
+
+      $timeout.flush(100);
+      expect(log).toEqual(['t1']);
+
+      $timeout.flush(1);
+      expect(log).toEqual(['t1', 't2']);
+    });
+
+
     describe('verifyNoPendingTasks', function() {
 
       it('should throw an exception when not flushed', function() {