Fixing several ShellCheck warnings

thirdparty/fetch-geoip-db.sh

@@ -5,7 +5,7 @@ set -e
 
 download_dir="${0%/*}/download"
 mkdir -p "${download_dir}"
-cd "${download_dir}"
+cd "${download_dir}" || exit 1
 
 filename="GeoLite2-Country.mmdb.gz"
 

thirdparty/fetch-thirdparty-deps-osx.sh

@@ -4,7 +4,7 @@ LAUNCHER_TAG="osx-launcher-20171118"
 
 download_dir="${0%/*}/download/osx"
 mkdir -p "$download_dir"
-cd "$download_dir"
+cd "$download_dir" || exit 1
 
 if [ ! -f libSDL2.dylib ]; then
 	echo "Fetching OS X SDL2 library from GitHub."

thirdparty/fetch-thirdparty-deps-windows.sh

@@ -6,7 +6,7 @@ set -e
 download_dir="${0%/*}/download/windows"
 
 mkdir -p "${download_dir}"
-cd "${download_dir}"
+cd "${download_dir}" || exit 1
 
 if [ ! -f SDL2.dll ]; then
 	echo "Fetching SDL2 from libsdl.org"

thirdparty/fetch-thirdparty-deps.sh

@@ -11,7 +11,7 @@ set -e
 download_dir="${0%/*}/download"
 
 mkdir -p "${download_dir}"
-cd "${download_dir}"
+cd "${download_dir}" || exit 1
 
 if [ ! -f StyleCopPlus.dll ]; then
 	echo "Fetching StyleCopPlus from NuGet"
@@ -79,7 +79,7 @@ if [ ! -f FuzzyLogicLibrary.dll ]; then
 	rm -rf FuzzyLogicLibrary
 fi
 
-if [ ! -f SDL2-CS.dll -o ! -f SDL2-CS.dll.config ]; then
+if [ ! -f SDL2-CS.dll ] || [ ! -f SDL2-CS.dll.config ]; then
 	echo "Fetching SDL2-CS from GitHub."
 	if command -v curl >/dev/null 2>&1; then
 		curl -s -L -O https://github.com/OpenRA/SDL2-CS/releases/download/20161223/SDL2-CS.dll
@@ -90,7 +90,7 @@ if [ ! -f SDL2-CS.dll -o ! -f SDL2-CS.dll.config ]; then
 	fi
 fi
 
-if [ ! -f OpenAL-CS.dll -o ! -f OpenAL-CS.dll.config ]; then
+if [ ! -f OpenAL-CS.dll ] || [ ! -f OpenAL-CS.dll.config ]; then
 	echo "Fetching OpenAL-CS from GitHub."
 	if command -v curl >/dev/null 2>&1; then
 		curl -s -L -O https://github.com/OpenRA/OpenAL-CS/releases/download/20151227/OpenAL-CS.dll