#35 don't try to clone wiki if there isn't one

[Anonymous]

Originally created by: thkoch2001

github-backup prints a rather ugly and misleading error message for any repository that does not have the wiki feature enabled:

Updating github_thkoch2001_epresent.wiki
fatal: remote error: access denied or repository not exported: /9/nw/95/76/56/965522/12167241.wiki.git
error: Could not fetch github_thkoch2001_epresent.wiki

It might be possible to check, whether a repository has the wiki feature enabled before trying to fetch it, see the 'has_wiki' property in the responses:
https://developer.github.com/v3/repos/#list-your-repositories