Better error reporting in case a computation fails

[azawlocki]

Currently it may look as follows (in the DEBUG log):

[2020-10-21 00:10:53,497 DEBUG yapapi.runner] ComputationFailed(reason='ApiException()')
[2020-10-21 00:10:53,497 ERROR yapapi.summary] Computation failed, reason: ApiException()

This is because we use __repr__() function on the exception that causes the failure. In case of ApiException(), __str__() would give us more useful information (but for certain other exceptions,
__str__() returns just the empty string).

Moreover, we don't store the exception info with ComputationFailed event.

Compare with how worker errors are reported:

[2020-10-21 15:07:28,698 DEBUG yapapi.executor] WorkerFinished(agr_id='241d4b5f768d226c5a6a06b2650b22826053fdb43ed0e060d09664527215e311', exc_info=(<class 'yapapi.rest.activity.CommandExecutionError'>, CommandExecutionError('stderr: Local service error: Transfer error: IO error: No such file or directory (os error 2)', 5), <traceback object at 0x7f5e65c36910>))
Traceback (most recent call last):
  File "/home/azawlocki/golem/yagna-integration/yapapi/yapapi/executor/__init__.py", line 354, in start_worker
    await command_generator.athrow(*sys.exc_info())
  File "examples/blender/blender.py", line 50, in worker
    yield ctx.commit()
  File "/home/azawlocki/golem/yagna-integration/yapapi/yapapi/executor/__init__.py", line 322, in start_worker
    async for step in remote:
  File "/home/azawlocki/golem/yagna-integration/yapapi/yapapi/rest/activity.py", line 137, in __aiter__
    raise CommandExecutionError(result.message, last_idx)
yapapi.rest.activity.CommandExecutionError: ('stderr: Local service error: Transfer error: IO error: No such file or directory (os error 2)', 5)

This is possible since the WorkerFinished event stores the whole traceback. We should use similar mechanism when reporting computation failures.