Fix: retain stream read error on subsequent response.content access

[hellozzm]

Summary

Fixes #4965

When accessing response.content raises an exception during stream reading (e.g., ConnectionError), subsequent accesses would silently return an empty bytestring b"" instead of re-raising the original error.

This made debugging difficult, especially in debuggers where properties may be accessed multiple times.

Changes

• Store the exception in _content_error when iter_content raises during content reading
• On subsequent accesses, re-raise the stored exception

Testing

from unittest.mock import MagicMock
from requests.models import Response

r = Response()
r.status_code = 200
r.raw = MagicMock()
r.raw.stream = MagicMock(side_effect=ConnectionError('reset'))

try:
    _ = r.content
except ConnectionError:
    print('First: raised')  # ✓

try:
    _ = r.content
except ConnectionError:
    print('Second: raised')  # ✓ (previously returned b'')