UI: Improve UI experience when user enters a wrong URL or link to a private repo

[aimanfatima]

Right now when you paste a git repo link to the input and try to choose a branch, behind the scenes it calls Github api to fetch all the branches for that repo.

If user doesn't enter correct link, or tries to enter link to a private repo the UI should show an error message saying "Please provide correct url" or "Please provide link to public repo or use our CLI if you would like to generate index for a private repo" respectively