[AppVeyor] Only display cygwin setup log when there is an error.

appveyor.yml

@@ -28,7 +28,7 @@ install:
     - 'git submodule update --init --recursive'
     # Install ocaml using wodi
     - appveyor DownloadFile "http://cygwin.com/setup-%CYG_ARCH%.exe" -FileName "%CYG_ROOT%\setup.exe"
-    - '%CYG_ROOT%/setup.exe -q -R "%CYG_ROOT%" -P dos2unix -P diffutils -P cpio -P make -P patch -P mingw64-%MINGW_ARCH%-gcc-core -P mingw64-%MINGW_ARCH%-gcc-g++'
+    - '%CYG_ROOT%/setup.exe -q -R "%CYG_ROOT%" -P dos2unix -P diffutils -P cpio -P make -P patch -P mingw64-%MINGW_ARCH%-gcc-core -P mingw64-%MINGW_ARCH%-gcc-g++ >log.txt || type log.txt'
     - '%CYG_ROOT%/bin/bash -lc "cygcheck -dc cygwin"'
     - '%CYG_ROOT%/bin/bash -lc "wget -q http://ml.ignorelist.com/wodi/8/wodi%WODI_ARCH%.tar.xz -O /tmp/wodi%WODI_ARCH%.tar.xz"'
     - '%CYG_ROOT%/bin/bash -lc "cd /tmp && rm -rf wodi%WODI_ARCH% && tar -xf wodi%WODI_ARCH%.tar.xz && bash wodi%WODI_ARCH%/install.sh"'