More detailed error message for 403 forbidden.

Currently, if user tried to access a git repository via HTTP and it fails because the user's permission is not enough to access the repository, git client tells that http request failed and the error was 403 forbidden.

But It is not enough for user to understand why it fails, especially if the user don't know the username because git-credential-osxkeychain authenticate implicitly without user knowing.

It would be much better if git client shows response body which might include an explanation of the failure. For example,

before:

$ git clone http://localhost/foo/bar error: The requested URL returned error: 403 while accessing http://localhost/foo/bar fatal: HTTP request failed

after:

$ git clone http://localhost/foo/bar error: The requested URL returned error: 403 while accessing http://localhost/foo/bar remote: User 'me' does not have enough permission to access the repository. fatal: HTTP request failed