Give a more accurate failure message when trying to unprotect; don't
complain about pages being write protected, just say that unprotect is
not supported by the hardware ... referencing the new "recover" command,
which is the way to achieve that.

Likewise, when trying to protect, talk about "pages" (matching hardware
doc) not "sectors" (an concept that's alien to these chips).

Also make the helptext for the "recover" command mention that it
also erases the device.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>