Error state indication for message with bitrate_switch=True

**Describe the bug**
Error state indication observed for can.Message with is_fd=True and bitrate_switch=True

**To Reproduce**
Steps to reproduce the behavior (please add longer code examples below):

If i create CAN bus with fd=True and create can.Message with is_fd=True
outgoing message printout looks ok:
ID: 0123 S F BS DLC: 8 ff ff ff ff ff ff ff ff

But if i try send can.Message with is_fd=True and bitrate_switch=True,
i get error indication
ID: 0123 S F BS EI DLC: 8 ff ff ff ff ff ff ff ff

**Expected behavior**
I expect that bitrate_switch defines BRS bit set or not,
and without any errors.
Or it can not be used to set BRS bit - just for indication?
What does it mean EI ?
What error can cause it?

**Additional context**
socketCAN used

No source file or test is named. Start by reproducing the socketCAN case with a CAN bus using fd=True and a can.Message using is_fd=True and bitrate_switch=True, then trace how the BRS and EI indicators are produced. Done means the meaning and cause of EI are established and the expected bitrate-switch behavior is corrected or clearly documented.

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