Ensure errors in sync periodic callbacks are logged.

[ceball]

Also: Do not duplicate tracebacks in logs for async periodic callbacks.

But: I don't really know what I'm doing :)

While I'm not yet a user of any recent version of Panel ( 😭 ), I tried to test the latest and found the current default situation seems to be:

• exceptions in async periodic callbacks are logged twice: once by panel (https://github.com/holoviz/panel/blob/main/panel/io/callbacks.py#L137), and once by bokeh (https://github.com/bokeh/bokeh/blob/branch-3.7/src/bokeh/util/tornado.py#L141)
• exceptions in sync callbacks are not logged at all (Make it easier to understand why periodic callback is not working #5540) because exceptions are silently caught by panel (https://github.com/holoviz/panel/blob/main/panel/io/callbacks.py#L89) and never make it to bokeh

I don't know if the desired (default) behavior is to have logging of errors in periodic callbacks come from panel, or bokeh, or both. I'll be happy to change this PR with guidance on the desired behavior!

Before this PR's change in panel/io/callbacks.py, the results of the tests added in this PR are:

• async: FAILED [...] assert 6 == 3 (because the error traceback appears twice per call)
• sync: FAILED [...] assert 0 == 3 (because the error never appears)

Note: I haven't yet gotten myself set up to be able to run all current Panel's tests successfully; I have not yet run tests beyond test_server, nor have I run test_server with fastapi.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.79%. Comparing base (8062670) to head (060a742).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7665      +/-   ##
==========================================
+ Coverage   86.29%   86.79%   +0.49%     
==========================================
  Files         346      346              
  Lines       52325    52349      +24     
==========================================
+ Hits        45153    45435     +282     
+ Misses       7172     6914     -258     

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[philippjfr]

Yay, great to hear from you!

But: I don't really know what I'm doing :)

Could have fooled me :)

While I'm not yet a user of any recent version of Panel

I'm sorry!

I don't know if the desired (default) behavior is to have logging of errors in periodic callbacks come from panel, or bokeh, or both. I'll be happy to change this PR with guidance on the desired behavior!

You're not missing anything, this is definitely the desired behavior.