Prevent warnings by adding isset() when accessing array indexes

The visualize_backtrace_get_lookup_table_values() function contains the $lookup_table color codes for all of the Drupal core filenames. But whenever there is a contributed module that isn't listed in the array, then there are warnings because isset() isn't being run first to determine if it exists.

I believe one existence of this is whenever $lookup_table[$via_filename]["source_code_line_url"] is called.

There are probably others as well.

Adding specific line numbers of warnings +module version number will help identify other instances.