[BUG] Binary ops fail unexpectedly for Decimal inputs

[bdice]

Describe the bug
cuDF 22.08 raises an UnboundLocalError without much helpful context when attempting to compute certain operators like __pow__ or __floordiv__ on decimal columns.

Steps/Code to reproduce bug

>>> from decimal import Decimal
>>> import cudf
>>> cudf.Series([Decimal("3.")]) ** 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/bdice/mambaforge/envs/rapids-22.08/lib/python3.9/site-packages/cudf/core/mixins/mixin_factory.py", line 11, in wrapper
    return method(self, *args1, *args2, **kwargs1, **kwargs2)
  File "/home/bdice/mambaforge/envs/rapids-22.08/lib/python3.9/site-packages/cudf/core/indexed_frame.py", line 2984, in _binaryop
    ColumnAccessor(type(self)._colwise_binop(operands, op)),
  File "/home/bdice/mambaforge/envs/rapids-22.08/lib/python3.9/contextlib.py", line 79, in inner
    return func(*args, **kwds)
  File "/home/bdice/mambaforge/envs/rapids-22.08/lib/python3.9/site-packages/cudf/core/frame.py", line 1707, in _colwise_binop
    else getattr(operator, fn)(left_column, right_column)
  File "/home/bdice/mambaforge/envs/rapids-22.08/lib/python3.9/site-packages/cudf/core/mixins/mixin_factory.py", line 11, in wrapper
    return method(self, *args1, *args2, **kwargs1, **kwargs2)
  File "/home/bdice/mambaforge/envs/rapids-22.08/lib/python3.9/site-packages/cudf/core/column/decimal.py", line 107, in _binaryop
    return result
UnboundLocalError: local variable 'result' referenced before assignment

Expected behavior
Either the operation will be computed successfully OR a helpful/informative error message is shown. The UnboundLocalError is indicative of a bug in the cuDF Python code but not necessarily an indication of whether libcudf can perform the requested computation.

Environment overview (please complete the following information)

• Environment location: Bare-metal
• Method of cuDF install: conda

Additional context
Discovered as part of #10178.

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