`install.sh`: detect a defined but nonexistent $NVM_DIR

(ref: #1700)

install.sh

@@ -294,6 +294,10 @@ nvm_check_global_modules() {
 }
 
 nvm_do_install() {
+  if [ -n "${NVM_DIR-}" ] && ! [ -d "${NVM_DIR}" ]; then
+    echo >&2 "You have \$NVM_DIR set to \"${NVM_DIR}\", but that directory does not exist. Check your profile files and environment."
+    exit 1
+  fi
   if [ -z "${METHOD}" ]; then
     # Autodetect install method
     if nvm_has git; then