[FixRM:#8132] - Fix undefined method '+' in total_commander.rb

The return value of registry_getvaldata can return nil when a
RequestError occurs, so you can't always assume it's gonna throw
you a string.
Avoid misleading error
Don't use begin/rescue to shut errors up and call it "file not found"
Use expand_path from Msf::Post::File
These funcs want 'filename'
Use check_other more
Land #2067, @wchen-r7's fix for total_commander
Fix path build on total_commander