python: usability improvements for `JournalConsumer` class

[grondo]

@wihobbs had pointed out some usability issues with the JournalConsumer class yesterday after the PR was merged. This PR tackles those issues.

• Document that poll() must be called after start().
• handle the above case better by raising a specific RuntimeException when the JournalConsumer isn't started before poll() is called
• allow set_callback() to be called before start() - there's no reason not to allow it.
• add tests accordingly.

[grondo]

Thanks @wihobbs! I actually just pushed one more doc update here: A note/warning about calling start() multiple times. I assume this is ok and will set MWP.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.52%. Comparing base (86db3b4) to head (cb3f4b6). Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
src/bindings/python/flux/job/journal.py	91.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #6390 +/- ## ======================================= Coverage 83.52% 83.52% ======================================= Files 525 525 Lines 87900 87907 +7 ======================================= + Hits 73420 73426 +6 - Misses 14480 14481 +1 ``` | [Files with missing lines](https://app.codecov.io/gh/flux-framework/flux-core/pull/6390?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework) | Coverage Δ | | |---|---|---| | [src/bindings/python/flux/job/journal.py](https://app.codecov.io/gh/flux-framework/flux-core/pull/6390?src=pr&el=tree&filepath=src%2Fbindings%2Fpython%2Fflux%2Fjob%2Fjournal.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework#diff-c3JjL2JpbmRpbmdzL3B5dGhvbi9mbHV4L2pvYi9qb3VybmFsLnB5) | `99.09% <91.66%> (+0.06%)` | :arrow_up: | ... and [11 files with indirect coverage changes](https://app.codecov.io/gh/flux-framework/flux-core/pull/6390/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework)