Use console instead of log in nuxeo-user-avatar element

The element nuxeo-user-tag is used to display a user avatar with this code:

 <nuxeo-user-tag fetch-avatar user="[[value]]"></nuxeo-user-tag>

If the user is not found, this error is displayed in the browse console

elements.html:46984 Uncaught (in promise) ReferenceError: log is not defined
    at HTMLElement.<anonymous> (elements.html:46984)
    at <anonymous>
(anonymous) @ elements.html:46984
...
elements.html:24097 

because of this code:

log.warn("Cannot fetch profile for user " + this._username(this.user));

The proper way to log a message in JavaScript is to use Console, not log

The route cause is getting an unknown user returns a 404, but it may not be possible to fix that

GET http://localhost:8080/nuxeo/api/v1/user/system 404 ()