[FIX] pad: improve pad bootstrap, prevent creating useless empty pads, avoid edition...

[FIX] pad: improve pad bootstrap, prevent creating useless empty pads, avoid edition warning in read-only mode for new pads, translate messages

Added a small method to detect the proper server configuration,
and make it degrade gracefully if the method is not present.
This avoids having to force a pad URL generation in order to
test the config (possibly creating useless pads).
Add a user-friendly message when the pad has not yet been
initialized for a given record, in read-only mode.

bzr revid: odo@openerp.com-20140314131849-rnjvk1pqpiyvtc1c