Add error-handling to GUI
Add error-handling to GUI

The GUI now displays a warning window and does not load a table file
if:
  - There is an issue with the formatting of the file e.g. number of
    columns, gaps.
  - The CSV reader is given bad input.
  or
  - The selected file(s) cannot be found.

whonut/Random-Table-Roller,whonut/Random-Table-Roller,whonut/Random-Table-Roller