Tell user that folder does not exist in current file structure

[jaredrkozar]

When running a program via gulp build, gulp dev, or gulp serve, if the dev, html, etc. folders do not exist, the terminal should tell the user that the folder does not exist instead of throwing a bunch of errors.

Additionally, you could ask the user the name of the folder they want to validate the html/css/javascript in (when the user first uses gulp dev, build, or serve) instead of always looking in `dev/html/', 'dev/js/', etc. What happens if the user wants to run the gulp file script in, say, a folder named myhtmlfolder? The user has to manually change every folder name in the gulpfile, which they might not want to do.