pldm_visualise_pdrs.py: add some error handling
pldm_visualise_pdrs.py: add some error handling

If pldmtool exits with non-zero status it will likely cause a json
decoder error to be thrown because it didn't write anything to stdout.
This is misleading, so check the pldmtool exit status before attempting
to read the json document it prints.

Change-Id: I8e6d1008a054bc6ed0699f051fb537be8785c863
Signed-off-by: Brad Bishop <713d098c0be4c8fd2bf36a94cd08699466677ecd@fuzziesquirrel.com>

openbmc/pldm,openbmc/pldm,openbmc/pldm