Add blurb to doc about "Must Have Error Callback"

[aaronfrost]

If you don't have an updater.on('error', fn) listener, all errors from the updater will bubble to the end users. Modify the documentation to include an explanation of this behavior. You must have the error handler, even if it is empty, it just has to exist.

[develar]

Project on NPM since version 1.0.0 is a part of electron-builder.