mhshow: suppress error for undisplayable parts, if they're not wanted

mhshow: suppress error for undisplayable parts, if they're not wanted

consider this message:

   1       multipart/mixed          1113K
     1     multipart/alternative      292
     1.1   text/html                   66
     1.2   text/plain                  49
     2     image/jpeg                823K

if this is displayed using "mshow -part 2", in the presence of an
MHSHOW variable which _only_ describes a rule for displaying images,
then we don't want to get an error about how nothing in part 1 was
displayable.