[Fix] nvm-exec: improve the no-version failure message

The message claimed "no .nvmrc file found" even when one was found and its version simply was not installed,
and it never mentioned that `NODE_VERSION=default` selects the `default` alias.

Refs #3810

nvm-exec

@@ -12,7 +12,9 @@ if [ -n "$NODE_VERSION" ]; then
 else
   { NVM_RC_VERSION="$(nvm_rc_version 3>&1 1>&4)"; } 4>&1 && nvm_ensure_version_installed "$NVM_RC_VERSION";
   if ! nvm use >/dev/null 2>&1; then
-    echo "No NODE_VERSION provided; no .nvmrc file found" >&2
+    nvm_err 'nvm-exec: unable to select a node version'
+    # shellcheck disable=SC2016
+    nvm_err '  Set `NODE_VERSION` (e.g. `NODE_VERSION=default`), or add an `.nvmrc` file.'
     exit 127
   fi
 fi