feat(python): provide better errors when failing to read CSV data from buffers that have advanced their read position

[alexander-beedie]

Ref: #6875.

The above PR inadvertently introduced a breaking change when reading CSV data from StringIO buffers. This patch doesn't undo that change, but instead significantly improves the errors raised when the breaking behaviour is triggered:

Example:

from io import StringIO
import polars as pl

s = StringIO("a,b\n1,2")
df1 = pl.read_csv(s, dtypes=[pl.Int32,pl.Float64])

# reading "s" again blows up (it didn't before),
# as the buffer's read position has been advanced
df2 = pl.read_csv(s, dtypes=[pl.Int8,pl.Float32])

Before: (raises a basic error)

# NoDataError: empty csv

After: (still raises, but now provides much more context as to why...)

# NoDataError: empty CSV data from StringIO (buffer position = 7; try seek(0) before reading?)