BUG: misleading message when using to_datetime with format='%V %a'

[MarcoGorelli]

• closes BUG: misleading message when using to_datetime with format='%V %a' #50308 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Good call @mroeschke to factor this out! I think it's simpler like this, and it helped find some untested code, and other code where the message was misleading

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Performance is unaffected:

On this branch:

In [1]: format = '%Y-%d-%m %H:%M:%S%z'

In [2]: dates = pd.date_range('1900', '2000').tz_localize('+01:00').strftime(format).tolist()

In [3]: %%timeit
   ...: pd.to_datetime(dates, format=format)
   ...: 
   ...: 
234 ms ± 470 µs per loop (mean ± std. dev. of 7 runs, 1 loop each)

upstream/main (after having recompiled):

In [1]: format = '%Y-%d-%m %H:%M:%S%z'

In [2]: dates = pd.date_range('1900', '2000').tz_localize('+01:00').strftime(format).tolist()

In [3]: %%timeit
   ...: pd.to_datetime(dates, format=format)
   ...: 
   ...: 
234 ms ± 885 µs per loop (mean ± std. dev. of 7 runs, 1 loop each)

[MarcoGorelli]

this one was never tested to begin with - have added a test to cover it

[MarcoGorelli]

I've put the "%Y and %V used together" check before the "%V used without both %G and weekday directive", as I think it produces a more useful error message in this case

[MarcoGorelli]

on upstream/main, this one would show "ISO week directive '%V' is incompatible with the year directive '%Y'", which is misleading because we didn't use "%Y"

[mroeschke]

Suggested change

	elif ("%V" in fmt and "%Y" in fmt):
	elif "%V" in fmt and "%Y" in fmt:

Nit

[mroeschke]

Thanks @MarcoGorelli