trap and report login errors

[jeff-zucker]

Traps and reports three kinds of errors when attempting to get a cookie:

• failed fetch
• fetched but login failed (400,no cookie)
• fetched, other bad response

I am assuming that a success returns 302.

[josephguillaume]

Looks good to me. Improves robustness/usability and only applies to getNodeSolidServerCookie, so it's something that won't be covered by other auth libraries.