Display error if Bun is installed

[mcmire]

When running :GithubPreviewStart, if Bun is not available, it would be good to know about it, and also how to install Bun (perhaps link to the Bun installation documentation). Otherwise all that happens is that a message github-preview: init is displayed, but nothing happens.

[owczr]

i had a same issue. it's easy to figure out, but the error message could be more informative