ACS:Added missing ERR header for constant value images
ACS:Added missing ERR header for constant value images

Fixes #299

Signed-off-by: Michele De La Pena <39e55d85e5271a056ecbbe31e099cc5784f7c8f7@stsci.edu>

jhunkeler/hstcal,jhunkeler/hstcal,jhunkeler/hstcal