Throw an error if no background is given to SimpleSlide.Builder

[JuliCodes]

Hey,

you developed a really great library.

There ist just one annoying thing for beginners. If i don't supply a background color to the SimpleSlide.Builder and run the generated code on any device (tested on 1+1 with CM13 and several emulators i got an RessourceNotFoundException.

My suggestion:

• Either documentate this behavior in the Readme
• Or throw a more helping message than the system does.

If wished, i can create a pull request.

Greetings

[janheinrichmerker]

Yeah, you can create a pull request for that. Look at FragmentSlide.Builder.build() on how this may look. (Think I have simply missed to implement this in SimpleSlide too 😄)