Add showControls, error handling, optimisations

Add a function to show button mappings using L + LEFT.
Added preliminary error handling that will be improved later to
determine playback issues.
Music is not stopped when an unsupported file is selected.
Some other minor modifications.

TODO:
Pause/play status does not show currently.
Obtain error status of playback thread.

Signed-off-by: Mahyar Koshkouei <deltabeard@users.noreply.github.com>

source/error.c

@@ -0,0 +1,29 @@
+#include "error.h"
+#include <errno.h>
+#include <string.h>
+
+char* ctrmus_strerror(int err)
+{
+	char* error;
+
+	switch(err)
+	{
+		case NDSP_INIT_FAIL:
+			error = "NDSP Initialisation failed";
+			break;
+
+		case DECODER_INIT_FAIL:
+			error = "Unable to initialised decoder";
+			break;
+
+		case FILE_NOT_SUPPORTED:
+			error = "File type is not supported";
+			break;
+
+		default:
+			error = strerror(err);
+			break;
+	}
+
+	return error;
+}