ci(windows): generate_win.bat — fail loudly when MSYS2 isn't installed (#6082)

Before: when `%MSYS2_DIR%` doesn't exist, the script printed an error
message and then exited 0, so callers (CI workflows, devs) thought
binding generation succeeded with no output.

After: print the error and `exit /b 1`. De-nest the success path so
the success branch is the script's normal flow rather than the
`else` branch of the missing-MSYS2 check.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Fedr

scripts/mrbind/generate_win.bat

@@ -15,7 +15,8 @@ set args=%*
 
 if not exist %MSYS2_DIR% (
     echo MSYS2 was NOT found at `%MSYS2_DIR%`. Run `install_deps_windows_msys2.bat` to build it.
-) else (
-    echo Found MSYS2 at `%MSYS2_DIR%`.
-    call %MSYS2_DIR%\msys2_shell.cmd -no-start -defterm -full-path -here -clang64 -c "time make -f '%~dp0generate.mk' %args:"=""% "
+    exit /b 1
 )
+
+echo Found MSYS2 at `%MSYS2_DIR%`.
+call %MSYS2_DIR%\msys2_shell.cmd -no-start -defterm -full-path -here -clang64 -c "time make -f '%~dp0generate.mk' %args:"=""% "