feat: allow for tests to be run without Polars installed, unify exceptions more

[mgorny]

CC @MarcoGorelli

Sending early to get feedback on the approaches used.

I went for:

1. pytest.importorskip() if the whole test (or module) seemed to rely on polars.
2. Explicit try/except ImportError: for tests that partially rely on polars — figured it would be weird to have them reported as "skipped" when the other part passed.
3. An explicit tuple variable for expected exceptions for pytest.raises(), with appending the polars exception when polars constructor is used.

Issue #1726

[MarcoGorelli]

thanks @mgorny ! your approach looks good, will take a closer look at the weekend

[MarcoGorelli]

ah, i tried unifying exceptions whilst we're here, but that got a bit out of hand - will revert and keep this to the original scope

[mgorny]

I'm sorry for not taking it further — it's been a busy week.

[MarcoGorelli]

no worries at all! will pick this up again for the next release