Resource test for blocking breaks intermittently on Travis

This test intermittently seems to break on Travis, on both 0.8 and 0.10 of node:

npm_lazy/test/resource.test.js

Line 486 in ad5acbb

assert.ok(!err);

I've updated the tests file to give the error output in the assert failures so we can see a bit better what's going on.

Failure example: https://travis-ci.org/CL0SeY/npm_lazy/jobs/43973760#L599

  1) resource tests tar resource with granular control when the resource is already fetching, block all pending requests:
     Uncaught 
  AssertionError: Error: URL is not in the npm_lazy cache, and it cannot be fetched (max retries exhausted): http://registry.npmjs.org/remote-valid2
      at onDone (/home/travis/build/CL0SeY/npm_lazy/test/resource.test.js:486:18)
...

Same code, working example: https://travis-ci.org/CL0SeY/npm_lazy/builds/43973879

Looks like it might be a timing related issue - not sure if it's a problem with the tests or the blocking yet.