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/main.h

@@ -13,6 +13,8 @@
 /* Maximum number of lines that can be displayed */
 #define	MAX_LIST		27
 
+static void showControls(void);
+
 static int changeFile(const char* ep_file);
 
 /**