Prevented exponential growth of error messages by avoiding nesting of error messages
Instead, used a counter to track how many operations had been applied to a LazyFrame after an error
Renamed `ErrorState` to `ErrorStateEncounters` and made it crate-private
Renamed `ErrorStateSync` to `ErrorState` and removed its Deref impl so that the only way to access its data is through (crate-private) accessors, thus reducing the size of its public API

…slashes in output

Minor cleanup + commenting

Prevented exponential growth of error messages by avoiding nesting of error messages
Instead, used a counter to track how many operations had been applied to a LazyFrame after an error
Renamed `ErrorState` to `ErrorStateEncounters` and made it crate-private
Renamed `ErrorStateSync` to `ErrorState` and removed its Deref impl so that the only way to access its data is through (crate-private) accessors, thus reducing the size of its public API

Instead of leaving a `String` in place of `PolarsError`, leave in place
the original `PolarsError`. Then, when it's time to produce a new error,
we clone the original error if possible; and it's almost always
possible, as all `PolarsError` variants contain ErrString as their data
except one: `Io`. So just convert `Io` to `ComputeError` with an error
message saying so, otherwise reuse the `PolarsError` variant with a
cloned message. (This is based off, and utilizes, the method
`PolarsError::wrap_msg`.)

This way we can retain the "shape" of the original error and just
augment its message to indicate that it's already failed n times.

Also cleaned up the `err_msg.rs` test in anticipation of adding a bit
more to that file.

Now that errors aren't taken and replaced by a String - the original is
stored and cloned as needed - we can simplify the implementation.
Instead of NotYetEncountered and AlreadyEncountered, we can just have a
single struct with a counteer (no enum needed). This massively
simplifies the implementation
Also, simplified the implemtnation of tests
Also cleaned up ErrorState's Debug implementation so that users don't
see the nuts and bolts inside the struct - they only see the important
debugging info

…e display of Python error messages