Report splice/write errnos, but revert splice fallback

The previous change helped to understand the acutal issue:
EPIPE because our wrapped program did not handle EAGAIN,
see https://github.com/libsndfile/libsndfile/pull/820

This commit reverts 51c0e396
in the sense to not fall back (because the EPIPE would then
be seen after the write()), but improves error reporting.

elbestream.c

@@ -297,6 +297,8 @@ splicer_fallback(const int *fds)
 		if (r == 0)
 			break;
 		r = write(fds[1], buf, (size_t)r);
+		if (r < 0)
+			perror("fallback write error");
 		assert(r >= 0);
 		assert(r >= 0);
 	}
@@ -318,11 +320,9 @@ splicer(void *a)
 	do {
 		r = splice(fds[0], NULL, fds[1], NULL, SIZE_MAX,
 		    SPLICE_F_MORE | SPLICE_F_MOVE);
-		if (r == -1) {
-			if (errno != EINVAL)
-				perror("splicer unexpected error");
+		if (r == -1 && errno == EINVAL)
 			return (splicer_fallback(fds));
-		}
+		perror("splicer unexpected error");
 		assert(r >= 0);
 	} while (r != 0);