Raise exception when no plcproj or sln file is found

[Roald87]

If you format in safe mode and it either fails to find a . plcproj file and/or a .sln file it will still do the formatting.

Make it such that the user receives an error message and it aborts the formatting.

• [x] Program should handle the file not found exception
• [x] Add test for plcproj not found
• [x] Add test for sln file not found