add checking for invalid entries in tag files safely parse the differents tag files, output warning when parsing an invalid entry.

Adapted behaviour of ui.username() to documentation and mention it explicitly: Searched in this order: $HGUSER, [ui] section of hgrcs, $EMAIL and stop searching if one of these is set. Abort if found username is an empty string to force specifying the commit user elsewhere, e.g. with line option or repo hgrc. If not found, use $LOGNAME or $USERNAME +"@full.hostname".