watchpoint_add() cleanup

Fail watchpoint_add() if it's the same address but the
parameters are different ... don't just assume having
the same address means the same watchpoint!  (Note that
overlapping watchpoints aren't detected...)

Handle unrecognized return codes more sanely; don't exit()!
And describe command params right.

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