Debugging output when subprocess errors...

[mgajda]

Currently pandoc-plot shows the script when the program errors.
Showing the error output instead would make it much faster to debug:
one can just use pandoc as a single WYSIWYG debugging environment,
which shows errors whenever output is unsatisfactory.

If this change of behaviour is puzzling, we can add debug option,
but I suggest that it should be the default for easy debugging.

[mgajda]

@LaurentRDC Do you think current output is satisfactory?
If not, is this something that may help and is worth implementing?
Is the description clear enough?