Include GUC's unit, if it has one, in out-of-range error messages.

 Include GUC's unit, if it has one, in out-of-range error messages.
 
 This should reduce confusion in cases where we've applied a units
 conversion, so that the number being reported (and the quoted range
 limits) are in some other units than what the user gave in the
 setting we're rejecting.
 
 Some of the changes here assume that float GUCs can have units,
 which isn't true just yet, but will be shortly.
 
 Discussion: https://postgr.es/m/3811.1552169665@sss.pgh.pa.us