Proper error handling in examples

All the unwrap()s are ugly and we can easily do better.

Idea would be to have main be a function that just calls run or a couple of other functions returning a Result<_,_> and it's only job is to destructure that and print any possible errors.

For each example we would then define an error enum (that implements Display, Error and what else) like

enum MyLittleError {
    MissingElement(&str),
    LinkingFailed(&str, &str),
    Other(&str),
}

and return those from the functions, ideally making use of the ?-operator and various Option<_> / Result<_, _> combinators, e.g. gst::ElementFactory::make("appsrc", None).ok_or(MyLittleError::MissingElement("appsrc"))?;.