exit if cd fails

.appveyor.sh

@@ -1,8 +1,8 @@
 set -x
-cd ${APPVEYOR_BUILD_FOLDER}
+cd "${APPVEYOR_BUILD_FOLDER}" || exit 1
 git submodule update --init --recursive
 mkdir -p build
-cd build
+cd build || exit 1
 export PATH="/mingw64/bin:/usr/local/bin:/usr/bin:/bin:/c/WINDOWS/system32:/c/WINDOWS:/c/WINDOWS/System32/Wbem:/c/WINDOWS/System32/WindowsPowerShell/v1.0:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl"
 exec bash -c "set -x && cmake -G\"MSYS Makefiles\" -DCMAKE_INSTALL_PREFIX=/mingw64 -DBUILD_TESTING=1 .. && make && make test"