Rescue Errno::EPERM

The api we call to get task info will raise this if you don't have
permission (eg trying to get info about another user's processes
when not root). By returning nil we'll fall back to the base case
of shelling out to `ps` (which doesn't have this issue because `ps` is
installed setuid & with the correct entitlements for this)

lib/get_process_mem.rb

@@ -120,5 +120,7 @@ def ps_memory
 
   def darwin_memory
     Darwin.resident_size(pid)
+  rescue Errno::EPERM
+    nil
   end
 end