Fix README for .exception_class

[tomkins]

Tiny README tweak, because results don't have an exception:

In [6]: result = default_task_backend.get_result("7d9d8e82-e62c-4fb2-8305-8e79b24d77c6")

In [7]: result.exception
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[7], line 1
----> 1 result.exception

AttributeError: 'TaskResult' object has no attribute 'exception'

In [8]: result.exception_class
Out[8]: subprocess.CalledProcessError

[RealOrangeOne]

D'oh - thanks!