Collect better error messages from failures

Sometimes failures can give us results like:

Traceback (most recent call last):
  File "/home/jenkins/workspace/pull-requests-benchmarks-benchmark/tests/benchmark_decision_tree.py", line 261, in test_RunMetrics
    self.assertTrue(result["Runtime"] > 0)
TypeError: 'int' object is not subscriptable

with stdout

[FATAL] Could not execute command:
orkspace/pull-requests-benchmarks-benchmark/libraries/bin/mlpack_decision_tree',
'-t', 'datasets/iris_train.csv', '-T', 'datasets/iris_test.csv', '-v', '-p',
'mlpack_dct_predict.csv'] The error is: Command
orkspace/pull-requests-benchmarks-benchmark/libraries/bin/mlpack_decision_tree',
'-t', 'datasets/iris_train.csv', '-T', 'datasets/iris_test.csv', '-v', '-p',
'mlpack_dct_predict.csv']' returned non-zero exit status -6

It would probably be helpful if we could provide the actual stdout or stderr output from the subprocess calls.

Start by tracing the subprocess calls used by the benchmark scripts, using tests/benchmark_decision_tree.py and its failure output as the example. Confirm how non-zero exit statuses are handled, then run the affected benchmark test and verify that relevant stdout or stderr is included in the reported failure.