Improved AbstractBasic.php log messages

There are two possible log messages in function authenticate.

a.) The current message text "No basic authentication headers were found" doesn´t give a clear statement what is going on, if there is a problem or not. According to a explanation of @evert, this can also be triggered when a client (eg a browser) tries to login and with this first attempt no headers are sent. Then they negotiate and with the second attempt the authentication method negotiated is used. Therefore I adopted this message to make this negotiating process more visible and better understandable.

b.) The second message just tells that there is a mismatch between a user and a password given. Currently no information is logged with which user this attempt was made. I extended the message text to printout the user used making it easier finding login related issues.