Simplified return value in Lib.Model.isValid()

I was wondering why we loop over @errors if we're only interested in how many errors there are. Then I realized it might be because Object.keys(someObj).length wouldn't work in IE8?

The change below makes this function compile into more lines in pure JS but would help to clarify the intent of the method. A for loop that will always break on the first element could be a bit confusing.