Migrate error handling to utils; ref logIn check.

Add:
	utils.py
		-create loggedIn() function to abstract the "'username' in
		session" conditional originally used throughout app.py; greater
		readability, cleaner source.

Ref:
	utils.py
		-most functions now return a boolean denoting either success or
		failure, and set an error message in app.session when
		appropriate.

	app.py
		-error handling now handled by utils.py, which stores error
		messages in a session["error"] variable: fewer ugly, nested
		conditionals.