kernel32: give a clearer message on 64-bit kernels when attempting to run DOS EXEs without Dosbox installed

[Austin English]

-- 
-Austin
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diff --git a/dlls/krnl386.exe16/wowthunk.c b/dlls/krnl386.exe16/wowthunk.c
index feb882b..0107662 100644
--- a/dlls/krnl386.exe16/wowthunk.c
+++ b/dlls/krnl386.exe16/wowthunk.c
@@ -39,6 +39,7 @@
 WINE_DEFAULT_DEBUG_CHANNEL(thunk);
 WINE_DECLARE_DEBUG_CHANNEL(relay);
 WINE_DECLARE_DEBUG_CHANNEL(snoop);
+WINE_DECLARE_DEBUG_CHANNEL(winediag);
 
 /* symbols exported from relay16.s */
 extern DWORD WINAPI wine_call_to_16( FARPROC16 target, DWORD cbArgs, PEXCEPTION_HANDLER handler );
@@ -564,6 +565,7 @@ BOOL WINAPI K32WOWCallback16Ex( DWORD vpfn16, DWORD dwFlags,
             if (errno != 0)  /* enter_vm86 will fall with ENOSYS on x64 kernels */
             {
                 WARN("__wine_enter_vm86 failed (errno=%d)\n", errno);
+                ERR_(winediag)("vm86 mode does not work on 64-bit kernels\n");
                 if (errno == ENOSYS)
                     SetLastError(ERROR_NOT_SUPPORTED);
                 else