Get rid of RET_SYS, just use RET_FAIL

Get rid of RET_SYS, just use RET_FAIL

RET_SYS was used for commands failing due to internal or environmental
errors, but not really systematically.  The difference to RET_FAIL is
how dispatch() treats them: RET_SYS got logged, and cost no BTUs.

More specific logging is possible at the point of failure than in
dispatch().  Make sure that's done for all failures that used to
return RET_SYS.

The change in BTU charging affects commands consider, offer, repay,
trade failing due to internal errors.  It also affects deity commands
reload and turn (irrelevant because deities get unlimited BTUs), and
commands apropos, info and motd (irrelevant because they cost no
BTUs).