GWT: x.x, hyperlink and URL command error message

If command id does not exist then display an error message in the status line.

The "xxx" command is unknown.
La commande "xxx" est inconnue. 

Validation test

• follow the link: http://portal.textometrie.org/test/?command=documentationx&path=%2FGRAAL&locale=fr
  • The message: La commande “documentationx” est inconnue. is displayed in the status bar
• follow the link: http://portal.textometrie.org/test/?command=documentationx&path=%2FGRAAL&locale=en
  • the message : The command “documentationx” is unknown. is displayed in the status bar
• follow the link: http://portal.textometrie.org/test/?command=documentation&path=%2FGRAAL&locale=fr
  • Graal home page is displayed
• click on the the link “Concordances” after “Obsolète”
  • The message: La commande “concordances” est inconnue. is displayed in the status bar

(from redmine: issue id 1119, created on 2014/11/12 by Matthieu Decorde)