`HTTPResponse._raw_read` raises `IncompleteRead` if the content length does
not match the expected content length. For malformed responses (e.g. 204
response with content) the re-raised `ProtocolError` was a bit too unclear
about the unexpected excess content being the reason for the exception.

With this change, the exception points out, that the client is not dealing
with a connection error but a protocol violation.

Closes urllib3#3261

…eteRead`