[python] Trivial rename within a unit-test file

[johnkerl]

Issue and/or context: As tracked on issue #2407 / [sc-51048].

Changes:

• There are places which currently say soma._exception.SOMAError, which is no longer necessary
• New spots, I want to type soma.SOMAError
• That would either be:
  • inconsistent, or,
  • it would be a distracting line-count addition on the WIP PR (which I am whittling down bit by bit in the form of split-out diffs)
• This little PR changes soma._exception.SOMAError to soma.SOMAError across the board in the apis/python/tests directory.

Notes for Reviewer: