Handle EMFILE errors when requiring modules

It is possible (at least on node 0.10.28) to get EMFILE errors thrown on webpack compilation. The errors originate inside of a `require` call.

Although one alternate solution is to increase the OS file limit, I don't believe that is a burden that should be pushed to the developer. On large projects, it is quite easy to hit the default OSX file descriptor limit.
Could you check for `instanceof Error`? We don't want to crash when stupid users throw non-`Error`s.