Git LFS plugin does not handle credentials

[dragly]

The Git LFS plugin doesn't handle cases where Git prompts the user to enter her credentials. We need to detect this case and issue a useful error telling the user to use ssh keys or store credentials in a way that avoids the prompt.

[alejoe91]

Git LFS fails if credeintials are not cached, because the Popen asks for credentials. One way of dealing with it could be caching credentials once and for all. (https://stackoverflow.com/questions/5343068/is-there-a-way-to-cache-github-credentials-for-pushing-commits)