stm32prog: add partition name in treat_partition_list error messages

Add the partition name and remove the line number in error messages
of treat_partition_list() to provide correct information to user of
STM32CubeProgrammer.

The "line number" value was confusing because it is incorrect here;
the index in  part_array[] is not aligned with the line number in
the parsed Layout file, because the empty lines and the lines beginning
by '#' are skipped during the first parsing in parse_flash_layout().

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>