improve pg_restore warning on text dump input

 From time to time there are complaints because people mistakenly feed a 
text format dump to pg_restore and get back a somewhat cryptic message 
about the file not being a valid archive. It's been suggested that we 
should have pg_restore run the file through psql, but that would involve 
more work than I at least care to give the problem. However, I think we 
should give a nicer message, suggesting the user try feeding the file to 
psql instead. The attached small patch does that.

cheers

andrew