NAME:
     flx2mag
 PURPOSE: (one line)
     Convert from flux units to magnitudes with errors.
 DESCRIPTION:
 CATEGORY:
     Photometry
 CALLING SEQUENCE:
     flx2mag,flux,fluxerr,mag,magerr
 INPUTS:
     flux    - Flux values for the magnitudes.
     fluxerr - Uncertainties on the fluxes.
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
     ZEROPT  - Magnitude that corresponds to a flux of 1. (default=0)
 OUTPUTS:
     mag    - Magnitudes (set to 99.9 if flux <= 0).
     magerr - Uncertainties on the magnitudes (set to 9.9 if flux <= 0).
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  2007/10/29, MWB, modifications to make it more error tolerant