add not found handler to prevent gulp interrupt when partial not found

I add `include` support yesterday, but when I type "wrong partial filename" and ejs will throw a error.
Now, gulp-ejs will catch this error and return PluginError and gulp interrupt.

But I think work flow interrupt by this is very not comfortable, and I didn't want to restart gulp every time.
So I add `notFoundHandler` to handle `ENOENT` error and log it to prevent gulp interrupt.
(But I am not sure this is fit gulp suggest, I just think this may improve gulp-ejs experience.)