Convert io.ErrUnexpectedEOF to a codes.Internal-marked status in toRPCerr. (#2228)

This is consistent with how io.ErrUnexpectedEOF is handled in a bunch of
other places (e.g. stream.go, server.go), and signals to
application-level code that the error is retryable.

rpc_util_test.go

@@ -179,6 +179,7 @@ func TestToRPCErr(t *testing.T) {
 	}{
 		{transport.StreamError{Code: codes.Unknown, Desc: ""}, status.Error(codes.Unknown, "")},
 		{transport.ErrConnClosing, status.Error(codes.Unavailable, transport.ErrConnClosing.Desc)},
+		{io.ErrUnexpectedEOF, status.Error(codes.Internal, io.ErrUnexpectedEOF.Error())},
 	} {
 		err := toRPCErr(test.errIn)
 		if _, ok := status.FromError(err); !ok {