Improve feedback for trying to bribe the watch

Improve feedback for trying to bribe the watch

Members of the watch can never be bribed, no matter how much gold is
thrown at them, but when a bribe was attempted they would say "that's
not enough" (implying incorrectly some higher amount would be enough to
pacify them).  Also, an already-peaceful mercenary would take on an
unusually aggressive tone when given any amount of gold, giving the
player the same message to "beat it" as when a bribe succeeds in
pacifying them.

Adjust the mercenary bribery code a bit so that a more friendly message
is given if the monster is already peaceful, and so that unbribeable
monsters don't imply they are just not being given enough gold.  I think
the actual effects of bribery shouldn't be affected by this commit, just
the messages/feedback.