feat: Add POLARS_BACKTRACE_IN_ERR for debugging

[orlp]

You can now specify POLARS_BACKTRACE_IN_ERR=1 to include a Rust backtrace inside errors to see where they get created:

>>> import polars as pl
>>> df = pl.DataFrame({"x": [1]})
>>> df.select(pl.col.y)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/orlp/programming/rust/polars/py-polars/polars/dataframe/frame.py", line 8853, in select
    return self.lazy().select(*exprs, **named_exprs).collect(_eager=True)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/orlp/programming/rust/polars/py-polars/polars/lazyframe/frame.py", line 2034, in collect
    return wrap_df(ldf.collect(callback))
                   ^^^^^^^^^^^^^^^^^^^^^
polars.exceptions.ColumnNotFoundError: y

Rust backtrace:
   0: std::backtrace_rs::backtrace::libunwind::trace
             at /rustc/7120fdac7a6e55a5e4b606256042890b36067052/library/std/src/../../backtrace/src/backtrace/libunwind.rs:116:5
   1: std::backtrace_rs::backtrace::trace_unsynchronized
             at /rustc/7120fdac7a6e55a5e4b606256042890b36067052/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2: std::backtrace::Backtrace::create
             at /rustc/7120fdac7a6e55a5e4b606256042890b36067052/library/std/src/backtrace.rs:331:13
   3: <polars_error::ErrString as core::convert::From<T>>::from
             at /Users/orlp/programming/rust/polars/crates/polars-error/src/lib.rs:49:17
   4: <T as core::convert::Into<U>>::into
             at /rustc/7120fdac7a6e55a5e4b606256042890b36067052/library/core/src/convert/mod.rs:759:9
   5: polars_plan::plans::aexpr::schema::<impl polars_plan::plans::aexpr::AExpr>::to_field_impl::{{closure}}::{{closure}}
             at /Users/orlp/programming/rust/polars/crates/polars-plan/src/plans/aexpr/schema.rs:78:60
...

One caveat: we now cache POLARS_PANIC_ON_ERR which already existed the first time we create an error object. So it's no longer supported to update that dynamically through os.environ.

[codecov]

Codecov Report

Attention: Patch coverage is 46.66667% with 8 lines in your changes missing coverage. Please review.

Project coverage is 80.42%. Comparing base (cdc741d) to head (326400c).
Report is 16 commits behind head on main.

Files	Patch %	Lines
crates/polars-error/src/lib.rs	46.66%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18333      +/-   ##
==========================================
- Coverage   80.48%   80.42%   -0.06%     
==========================================
  Files        1501     1492       -9     
  Lines      199457   198655     -802     
  Branches     2837     2837              
==========================================
- Hits       160541   159777     -764     
+ Misses      38390    38352      -38     
  Partials      526      526              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.