Iterator should return without errors if no more blocks

[s1na]

Propagating errors in iterator has caused an issue that I unfortunately hadn't foreseen. I'm sorry about this.

It tries to get the next blocks, and when the next block doesn't exist, it returns the NotFoundError upstream, which shouldn't be the case. I tried to fix this with minimal change , however please let me know if you want it fixed in another way.

Furthermore the PR modifies the test case checking for iterating an empty blockchain to check for errors in callback.

[coveralls]

Coverage increased (+0.009%) to 97.938% when pulling 083b617 on s1na:fix/iterator-err-handling into 8100e62 on ethereumjs:master.

[holgerd77]

Tested this locally on a local geth install, works. Wondered if this could be written in a bit more elegant way, but I don't want to spend too much more back-and-forth on this, so I will merge for now.

Will directly do a bugfix release afterwards.