Bugfix: Encoding issue error messages

In real life tests there were issues with cell values containing
non-ASCII characters. As Python does not know the encoding of Excel
input values, it assumes ASCII (0-128) when converting unicode to
str. It's set explicitly now.

groundwork_spreadsheets/patterns/ExcelValidationPattern/excel_validation_pattern.py

@@ -667,7 +667,7 @@ def _get_sheet(self, workbook):
 
     def _raise_value_error(self, msg):
         self._plugin.log.error(msg)
-        raise ValueError(msg)
+        raise ValueError(msg.encode('UTF-8', 'ignore'))
 
     def _transform_coordinates(self, row=None, column=None):
         if row is None and column is None: