Plater: Display a decent error message when loading a file fails

In plater, when loading a corrupted STL file, it displays traceback
in terminal but says nothing. This makes sure that it will display an actuall
GUI error message to the user.

Also added it to the SCAD load option.

Since pronterserve.py has been moved the path needs to be corrected.

Conflicts:
	printrun/projectlayer.py

Conflicts:
	printrun/projectlayer.py