make config validation failures clearer

[g-k]

see https://github.com/mozilla-services/buildhub/pull/393

Any of the following would be helpful:

• highlighting the importance of the whitespace in the "pin directive per file" example on https://pyup.io/docs/bot/config/
• handling NoneType items in the RequirementConfig constructor https://github.com/pyupio/pyup/blob/060c7f19bba45c1cb1a1d8f596775dceea94ea3a/pyup/config.py#L62
• more informative validation errors after adding a repo on the website
• a pyup config validator cli tool or a config validator website

[rafaelpivato]

I would love that. YAML config parsing needs better handling indeed.