Give clear error message when jQuery is missing

Created by: cvrebert

To avoid bug reports like #8547 (closed) (and having been confused for a minute myself when accidentally forgetting to include jQuery a couple times; Chrome's built-in $ in the debug console really doesn't help), I'd like to suggest including a tiny bit of JS (in the combined bootstrap.js only) along the lines of:

if (!jQuery) {
    throw new Error("Bootstrap's JavaScript requires jQuery, but jQuery isn't loaded.");
}

to make the error perfectly clear and apparent.