fix: only format exception for primary context

[agoose77]

We were previously formatting exceptions for each context manager, which is clearly wrong! (oops)

This PR course corrects, and ensures that only the primary context actually tries to format the exception.

Fixes #2386

[codecov]

Codecov Report

Merging #2388 (988ad34) into main (be60b53) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/_errors.py	80.00% <50.00%> (-0.49%)	⬇️

[jpivarski]

It's fixed!

>>> import awkward as ak
>>> no_fields = ak.Array([[{}, {}], [], [{}]])
>>> no_fields["new_field"] = 123
Traceback (most recent call last):
  File "/home/jpivarski/irishep/awkward/src/awkward/highlevel.py", line 1025, in __setitem__
    self._layout = ak.operations.with_field(
  File "/home/jpivarski/irishep/awkward/src/awkward/operations/ak_with_field.py", line 47, in with_field
    return _impl(array, what, where, highlevel, behavior)
  File "/home/jpivarski/irishep/awkward/src/awkward/operations/ak_with_field.py", line 87, in _impl
    raise ValueError("no tuples or records in array; cannot add a new field")
ValueError: no tuples or records in array; cannot add a new field

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

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/jpivarski/irishep/awkward/src/awkward/highlevel.py", line 1028, in __setitem__
    self._numbaview = None
  File "/home/jpivarski/irishep/awkward/src/awkward/_errors.py", line 54, in __exit__
    self.handle_exception(exception_type, exception_value)
  File "/home/jpivarski/irishep/awkward/src/awkward/_errors.py", line 64, in handle_exception
    raise self.decorate_exception(cls, exception)
ValueError: no tuples or records in array; cannot add a new field

This error occurred while calling

    ak.Array.__setitem__(
        self = <Array [[{}, {}], [], [{}]] type='3 * var * {}'>
        field_name = 'new_field'
        field_value = 123
    )

(And the above wouldn't even be an error in main.)