extproc: retrieve correct error status when Send fails with io.EOF (#9270)

Fixes: https://github.com/grpc/grpc-go/issues/9269

When the proc server fails abruptly , if it fails before the Send for
request header is called , we will get an io.EOF error which needs to be
retrived using Recv.
If the proc server fails after the Send for request header, i.e. the
Send returns nil, the correct error will automatically be retrieved by
Recv run in the background loop in `recvFromProcServer`.
So , because of this race between the proc server failing and Send being
called for request header , the test was flaking.
We need to get the correct error when Send for header req fails with
io.EOF.

#ext-proc-a93

RELEASE NOTES: None