rgbasm: improve error message for failed INCBIN

src/asm/yaccprt1.y

@@ -1,8 +1,11 @@
 %{
+#include <ctype.h>
+#include <err.h>
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <ctype.h>
+#include <sysexits.h>
 
 #include "asm/symbol.h"
 #include "asm/asm.h"

src/asm/yaccprt3.y

@@ -267,6 +267,7 @@ include			:	T_POP_INCLUDE string
 					{
 						if( !fstk_RunInclude($2) )
 						{
+							fprintf(stderr, "Could not open file '%s' : %s\n", $2, strerror(errno));
 							yyerror( "File not found" );
 						}
 					}