[Fix] `nvm-exec`: properly exit when failing; improve error messages, suppress `nvm use --help` output

Fixes #997.

nvm-exec

@@ -5,9 +5,10 @@ DIR="$(command cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
 . "$DIR/nvm.sh" --no-use
 
 if [ -n "$NODE_VERSION" ]; then
-  nvm use $NODE_VERSION > /dev/null || (echo "NODE_VERSION not set" >&2 && exit 127)
-else
-  nvm use > /dev/null || (echo "No .nvmrc file found" >&2 && exit 127)
+  nvm use $NODE_VERSION > /dev/null || exit 127
+elif ! nvm use >/dev/null 2>&1; then
+  echo "No NODE_VERSION provided; no .nvmrc file found" >&2
+  exit 127
 fi
 
 exec "$@"