Invalid testset ID in run_tests method has poor error message

[Rutledge]

When client.run_tests is provided invalid testsetid/scoring config we should error with a understandable message:

client.run_tests(
input_testset_id=123, -> Invalid id results in JSON error
scoring_config_id=456, -> Invalid id results in JSON error
model_invocation=lambda prompt: answer_query(prompt), #Replace with your system
)