fattureCCSR: added filepicker and filedialog for input and output files. Started working on exceptions handling.

Signed-off-by: Ettore Dreucci <ettore.dreucci@gmail.com>

exc.py

@@ -0,0 +1,7 @@
+"""Define Python user-defined exceptions"""
+
+class Error(Exception):
+    """Base class for other exceptions"""
+
+class WrongFileExtension(Error):
+    """Raised when a file extension is not accepted"""