Improve warning you get when attempting to view a log file you don't have access to

[TheWitness]

Describe the bug

If for some reason you don't have read access to one of the Cacti log files, issue a message that appears in the content area of the panel instead of in an area not intended for messages.

To Reproduce

Steps to reproduce the behavior:

1. Remove access to your stderr log files, say chmod 600 stderr;chown root:root stderr

2. Goto the 'Log' tab

3. Select one of the log files

4. Note that the not readable message is in the wrong part of the page

Expected behavior

Cacti should write content into the correct area within pages it renders.