Implement 12-bit-specific error/warn/trace macros

The macros in jerror.h refer to j_common_ptr, so it is unfortunately
necessary to introduce a 12-bit-specific version of that header file
(j12error.h) with 12-bit specific ERREXIT*(), WARNMS*(), and
TRACEMS*() macros.  (The message table is still shared between 8-bit and
12-bit implementations.)

Fixes #607

jerror.c

@@ -26,7 +26,7 @@
 #include "jinclude.h"
 #include "jpeglibint.h"
 #include "jversion.h"
-#include "jerror.h"
+#include "jerrorint.h"
 
 #ifdef USE_WINDOWS_MESSAGEBOX
 #include <windows.h>
@@ -48,7 +48,7 @@
 #define JMESSAGE(code, string)  string,
 
 const char * const jpeg_std_message_table[] = {
-#include "jerror.h"
+#include "jerrorint.h"
   NULL
 };