Don't error multiple times when trying to load missing default bus layout

[timothyqiu]

If "Load Default" is pressed when res://default_bus_layout.tres is missing, along with the "There is no 'res://default_bus_layout.tres' file." warning dialog, two errors are also printed in the output:

• Cannot open file 'res://default_bus_layout.tres'.
• Failed loading resource: res://default_bus_layout.tres. Make sure resources have been imported by opening the project in the editor at least once.

This PR gets rid of these two errors.

[akien-mga]

Thanks!