Updated sample

The sample now detects ProDOS file open errors in the main routine and
handles them appropriately.

I also found out that the carriage return character that I was using in
my PUTS strings should have been $8D and not $0D as I was using before.
While $0D worked in 80 column mode, it didn't in 40 column mode.

I have added code which can read one track of data from a nibble image.