Add better error handling for versions requests

Created by: eseliger

before:

15:53:15            frontend | WARN Failed to retrieve latest src-cli version, err: fetching src-cli release versions: json: cannot unmarshal object into Go value of type []srccli.releaseMeta

after:

15:56:32            frontend | WARN Failed to retrieve latest src-cli version, err: fetching src-cli release versions: Invalid response from GitHub: {"message":"API rate limit exceeded for XXX.XXX.XXX.XXX. (But here's the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.)","documentation_url":"https://developer.github.com/v3/#rate-limiting"}