[js preferences] fix various pb with js for preferences form: buggy clearPreviousError, bad prototype of callbacks given to validate form, dunno how error display was expecting to work...

+
−\.svn
+
−^build$
+
−^dist$
+
−\.pyc$
+
−\.pyo$
+
−\.bak$
+
−\.old$
+
−\~$
+
−\#.*?\#$
+
−\.swp$
+
−^doc/book/en/apidoc$