ErrorCallback option allows user to specify action to take for OS errors
ErrorCallback option allows user to specify action to take for OS errors

Right now there are two possible actions, the default, Halt, which
immediately stops processing items in the file system hierarchy, and
SkipNode, which skips the node that caused the error and proceeds to
the next file system node to be processed.

Resolves #2.

karrick/godirwalk,karrick/godirwalk