Suggest adding SSH key when none is present

[nikitavbv]

Currently if you try to clone a repository from github and you don't have any ssh keys configured at github, you will fail to clone and get such message:

Fatal error: fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

Even Permission denied (publickey). message is not shown.

@guyzmo offered to "suggest adding an SSH key when none is present"

[guyzmo]

well, we need to correctly catch the exception thrown by gitpython when it cannot pull from the remote, and eventually show the error (or a more litterate version of the error)

[why-Radhika]

Can you assign this issue to me..I would like to work on it as my first issue.