feat(python): raise more informative error message if someone lands on Expr.__bool__

[MarcoGorelli]

xref #14062

Landing on Expr.__bool__ is so common, because if expr: triggers expr.__bool__, and people often have no idea why they're seeing this error message at all

I'd suggest making the error message a bit rustier

Here's what the err message looks like now

TypeError: the truth value of an Expr is ambiguous

You probably got here by using a Python standard library function instead of the native expressions API.
Here are some things you might want to try:
- instead of `pl.col('a') and pl.col('b')`, use `pl.col('a') & pl.col('b')`
- instead of `pl.col('a') in [y, z]`, use `pl.col('a').is_in([y, z])`
- instead of `max(pl.col('a'), pl.col('b'))`, use `pl.max_horizontal(pl.col('a'), pl.col('b'))`

[stinodego]

I was a bit too quick to close there - indeed this is a better error message 👍