Return an error if --load-model is specified without explicit model control mode

[rmccorm4]

Before: Specifying --load-model without explicit model control mode will silently not work as expected, and instead load all models with the default model control mode.

root@ced35d0-lcedt:/opt/tritonserver# tritonserver --model-repository models --load-model my_model
...
I0522 18:28:47.526658 448 http_server.cc:4692] Started HTTPService at 0.0.0.0:8000

After: Specifying --load-model without explicit model control mode will fail, and prompt you to also set --model-control-mode=explicit. Parsing errors will also be reported at the bottom for immediate visibility.

root@ced35d0-lcedt:/opt/tritonserver# tritonserver --model-repository models --load-model abc
...
LONG
HELP
OUTPUT
...
# NEW: Log error for invalid arg, and log ParseException errors at the bottom as well for easier visibility
Error: Use of '--load-model' requires setting '--model-control-mode=explicit' as well.

NOTE: This error can be undone if/when we add support for --load-model in default model control mode.

[debermudez]

woohoo