Handle worker errors and timeout on 2i reformat

Before this fix, I noticed the reformat basically hang forever with a
crashed worker error once. This fix handles the error message from the
vnode worker and adds a big 2 minute timeout to make sure we don't get
stuck. In reality, it's mostly harmless, but annoying to leave a process
hanging forever like that.
Merge pull request #561 from basho/eas-2i-reformat-handle-error

Handle worker errors and timeout on 2i reformat