Less compile error should not break gulp

[grtjn]

Not sure this still occurs in latest slush, but worth a check. Add something like:

myclass {
  .label-warn;
}

to main.less, and you get this in older slush when running gulp less:

[11:30:36] Starting 'less'...

events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: .label-warn is undefined in file .../ui/app/styles/main.less line no. 179

After which gulp terminated abnormally. If running gulp watch, that would break too..

[grtjn]

Applies to the styles task in latest slush..