input/file: log message on error

If a file is removed the library, next time mpd will try to play it it
will result in an error 'ERROR: problems decoding some/file.ogg'.
Nothing is written in log files (verbose mode or not)

[mk: append strerror(errno)]

src/input/file_input_plugin.c

@@ -52,6 +52,8 @@ input_file_open(struct input_stream *is, const char *filename)
 	fd = open(pathname, O_RDONLY);
 	if (fd < 0) {
 		is->error = errno;
+		g_debug("Failed to open \"%s\": %s",
+			pathname, g_strerror(errno));
 		g_free(pathname);
 		return false;
 	}