Inability to match HTML files should give an error

[srmelody]

I had a configuration of useminPrepare that didn't match any HTML files. It would be great if the useminPrepare task would give an error / warning / log message that let me know this is why the downstream task configuration is undefined.

Running "useminPrepare:html" (useminPrepare) task
Going through  to update the config
Looking for build script HTML comment blocks
Configuration is now:

concat:
  { dist: {} }

uglify:
    undefined

cssmin:
    undefined

[sindresorhus]

Yup. PR welcome :)

[stephanebachelier]

Should be covered by #518