Fix error propagation in iterator

[s1na]

This fixes a minor issue where the iterator, after calling the run function, doesn't pass along its error to the callback.

[coveralls]

Coverage remained the same at 96.84% when pulling fcc98b1 on s1na:patch-1 into 3752761 on ethereumjs:master.

[vpulim]

@s1na Good catch. Thanks!

However, the fix still won't catch errors generated by _saveHeads() but something like this should do the trick:

(err) => err ? cb(err) : self._saveHeads(cb)

Also, it would be good to add a new test case for this to test/index.js. For example:

function iterateError (done) {
  blockchain.iterator('error', function (block, reorg, cb) {
    cb(new Error('iterator func error'))
  }, function (err) {
    t.ok(err, 'should catch iterator func error')
    done()
  })
}

[s1na]

@vpulim Sorry for the incomplete PR, and thanks for the help :) I've updated it with the points you mentioned.

[vpulim]

Great. Looks good to me!