nested if should be and

run

 #!/usr/bin/env sh
-set -e
+set -ex
 export PYTHONPATH="$(dirname $0):$PYTHONPATH"
 
 python_best_version() {
-  if [ -x "$(command -v python3)" ]; then
-    if python3 -c "import sys; sys.exit(not sys.version_info > (3, 5))"; then
+  if [ -x "$(command -v python3)" ] &&
+      python3 -c "import sys; sys.exit(not sys.version_info > (3, 5))"; then
     exec python3 "$@"
-    fi
   elif [ -x "$(command -v python2.7)" ]; then
     exec python2.7 "$@"
   else