Fix RayTaskRunner exception handling in Prefect >= 2.6.0

[rpeden]

In Prefect 2.6.0, exception_to_crashed_state changed from a sync function to an async function that needs to be awaited. RayTaskRunner does not currently await this call, resulting in users seeing coroutine' object has no attribute 'type' when an unhandled exception occurs while a Ray worker node is trying to unpickle and load the task.

Consequently, it is difficult to see and understand what went wrong. Looking at logs from the Ray worker node(s) that tried to execute the failed task(s) will usually show what went wrong, but not all users who can submit work to a Ray cluster will have access to individual worker node logs.

Several users have encountered this issue, as noted in this Discourse post and issue #58.

Changes in this PR:

• The task runner now unwraps the original exception whenever possible instead of showing a generic RayTaskError
• The task runner now awaits exception_to_crashed_state so users will see the actual exception instead of coroutine' object has no attribute 'result'
• Added a new test to help prevent future regression to previous behavior.

Closes #58

Example

Start by reproducing the error:

• Create a fresh Conda environment or venv
• run pip install prefect "ray[default]"
• Create a local Ray cluster by running ray start --head
• Run the following flow:

from prefect import task, flow
from prefect_ray import RayTaskRunner

task_runner = RayTaskRunner(
    address="ray://localhost:10001"
)

@task()
def test_task():
    raise KeyboardInterrupt()

@flow(task_runner=task_runner)
def test_flow():
    future = test_task.submit()
    future.result(10)

if __name__ == "__main__":
    test_flow()

• Notice the AttributeError: 'coroutine' object has no attribute 'result' error instead of the error you raised
• Pull in the changes from this PR
• Notice that you now see the actual error caused by the exception: ray.exceptions.TaskCancelledError: Task: TaskID(...) was cancelled

Checklist

• This pull request references any related issue by including "Closes #<ISSUE_NUMBER>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• This pull request includes tests or only affects documentation.
• Summarized PR's changes in CHANGELOG.md