file could reveal a little bit more when something goes wrong

Reference: /n/sources/patch/applied/file-diag
Date: Sat Feb  7 16:20:09 CET 2009
Signed-off-by: forsyth@terzarima.net

--- /sys/src/cmd/file.c	Sat Feb  7 16:19:26 2009
+++ /sys/src/cmd/file.c	Sat Feb  7 16:19:21 2009
@@ -260,7 +260,7 @@
 	}
 	fname = file;
 	if ((fd = open(file, OREAD)) < 0) {
-		print("cannot open\n");
+		print("cannot open: %r\n");
 		return;
 	}
 	filetype(fd);
@@ -346,7 +346,7 @@
 	/* may be reading a pipe on standard input */
 	nbuf = readn(fd, buf, sizeof(buf)-1);
 	if(nbuf < 0) {
-		print("cannot read\n");
+		print("cannot read: %r\n");
 		return;
 	}
 	if(nbuf == 0) {