Do you want to request a feature or report a bug?
A bug

What is the current behavior?
When in watch mode, if a file is busy when attempting to emit to it, watch stops functioning. The following error is appropriately logged:

Error: EBUSY: resource busy or locked, open '..\js\bundle.js'
    at Error (native)

If the current behavior is a bug, please provide the steps to reproduce.
run webpack --watch
save a watched file
output to a file which is currently being used by another process

What is the expected behavior?
An error is logged to the console but watch mode continues to function. This is the behavior of Grunt and Gulp watchers.

Please mention other relevant information such as the browser version, Node.js version, Operating System and programming language.
A valid reason why a file could be momentarily busy is if it is being synced to a server directory.