Authentication handling and error messages

[rtavcar]

Comments on login behaviour:

1. If user is configured in LDAP and is member of one of olog's groups, login is OK.
2. If LDAP is not configured, Olog returns a "wrong username or password" error.
3. When logging in as existing LDAP user, which is not in any olog group (olog-logs, olog-tags, olog-logbooks or olog-admins), Olog ignores the login and keeps "Guest" logged in. No login error is reported.
4. When logging in as bogus user, e.g. bogus:bogus, the browser's authentication pops up and infinitely requests the login data. No login error is reported by Olog.

Error message in case 2 is misleading. In cases 3 and 4, Olog should return meaningful error messages in the login screen. Message could be equal for both cases, something like "User unknown to olog."?

Cheers!

[berryma4]

@AnthonyDionise can you look at this please.