psql - add special variable to reflect the last query status

After some discussions about what could be useful since psql scripts now 
accepts tests, this patch sets a few variables which can be used by psql 
after a "front door" (i.e. actually typed by the user) query:

  - RESULT_STATUS: the status of the query
  - ERROR: whether the query failed
  - ERROR_MESSAGE: ...
  - ROW_COUNT: #rows affected

  SELECT * FROM ;
  \if :ERROR
    \echo oops
    \q
  \endif

I'm not sure that the names are right. Maybe STATUS would be better than 
RESULT_STATUS.

-- 
Fabien.