main: Be clear about what we failed to open

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I5884b83716f73da8f055a7267b3efab36c856ea8
diff --git a/main.c b/main.c
index d63d74c..a1b4953 100644
--- a/main.c
+++ b/main.c
@@ -548,7 +548,7 @@
 		devnode[PATH_MAX - 1] = '\0';
 
 		if ((sourcefd = open(devnode, O_RDONLY)) == -1)
-			err(EXIT_FAILURE, "Failed to open %s", devnode);
+			err(EXIT_FAILURE, "Failed to open source %s", devnode);
 
 		optind++;
 	}
@@ -565,7 +565,7 @@
 			 * then we patch this bit when we know what we need.
 			 */
 			if ((sinkfd = open(argv[optind], O_WRONLY)) == -1)
-				err(EXIT_FAILURE, "Failed to open %s", argv[optind]);
+				err(EXIT_FAILURE, "Failed to open sink %s", argv[optind]);
 
 			optind++;
 		}