show: Print type of MIME part instead of error message if not displayable.

show: Print type of MIME part instead of error message if not displayable.
A further step in the general rework of show/mhshow. Print less error
messages. Not being able to directly display some attachment is no
exceptional event, but quite normal. Therefore, no error message should
be printed but an information message that there is a MIME part.
Printing to stdout makes the message to be integrated into the normal
output.
There's more rework of show needed.