Add further info message that building the tree may have failed because of too little RAM baing available. We have see this a few times, either with way too many domains on devices like a Pi Zero or, e.g., only 256 MB RAM being allocated to VMs

Signed-off-by: DL6ER <dl6er@dl6er.de>

gravity.sh

@@ -97,6 +97,7 @@ gravity_build_tree() {
 
   if [[ "${status}" -ne 0 ]]; then
     echo -e "\\n  ${CROSS} Unable to build gravity tree in ${gravityTEMPfile}\\n  ${output}"
+    echo -e "  ${INFO} If you have a large amount of domains, make sure your Pi-hole has enough RAM available\\n"
     return 1
   fi
   echo -e "${OVER}  ${TICK} ${str}"