Fix a SystemError if unboxing fails in a ufunc

[eric-wieser]

Previously the conversion function could be called while an error was in flight, which isn't legal.

Discovered by #5422 which made this mistake a lowering error.

Output on master:

In [8]: f(arr, out)
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-6-bcca0651d5c8> in __int__(self)
      2     def __int__(self):
----> 3         raise RuntimeError()
      4

RuntimeError:

The above exception was the direct cause of the following exception:

SystemError                               Traceback (most recent call last)
<ipython-input-8-ec85c84fdad7> in <module>
----> 1 f(arr, out)

<ipython-input-6-bcca0651d5c8> in __int__(self)
      1 class BadType:
      2     def __int__(self):
----> 3         raise RuntimeError()
      4
      5

SystemError: <class 'RuntimeError'> returned a result with an error set

With this patch, the SystemError no longer occurs and RuntimeError happens by itself.

[stuartarchibald]

Thanks for submitting this PR to the Numba issue tracker, it's been added to the queue for review. Please be advised however that we are currently in a PR freeze as we are preparing to ship a release candidate (RC). We will begin reviewing PRs again once the RC is out and we thank you in advance for your patience!

[eric-wieser]

Any thoughts on this one?