Add helper to decode utf8 strings
Decode utf8 errors in printcore
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.
Merge pull request #436 from hroncok/master

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