Add err log for missing oathtool in Synology

Alerts the user that the oathtool is missing and the TOTP can't be
generated.

deploy/synology_dsm.sh

@@ -94,6 +94,11 @@ synology_dsm_deploy() {
 
   otp_code=""
   if [ -n "$SYNO_TOTP_SECRET" ]; then
+    if ! command -v oathtool &> /dev/null
+    then
+      _err "oathtool could not be found, install oathtool to use SYNO_TOTP_SECRET"
+      exit 1
+    fi
     otp_code="$(oathtool --base32 --totp "${SYNO_TOTP_SECRET}" 2>/dev/null)"
   fi