Prevent Looping when JX.Resource callbacks throw errors

Summary: When these two callsites of JX.Resource have callbacks that throw exceptions, we never change the state of the links variable and each time that _poll gets called, we run into the error again and again in a loop

Test Plan:
   JX.Resource.load([valid CSS resource that is not already loaded], function() { throw new Error('looper'); });

Reviewers: epriestley, hedger, CatChen

CC: aran, Korvin, piaw.na

Differential Revision: https://secure.phabricator.com/D4976