(core) Add CellError to explain when a formula refers to a cell with an error value

Summary:
get_cell_value wraps RaisedException with CellError to expand the error message for the user.

This is still pretty conceptual, the comments explain some things to think about, but it works and is an improvement.

Test Plan: Updated Python unit tests

Reviewers: dsagal, paulfitz

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2928

sandbox/grist/column.py

@@ -147,7 +147,12 @@ class BaseColumn(object):
     """
     raw = self.raw_get(row_id)
     if isinstance(raw, objtypes.RaisedException):
-      raise raw.error
+      if isinstance(raw.error, depend.CircularRefError):
+        # Wrapping a CircularRefError in a CellError is often redundant, but
+        # TODO a CellError is still useful if the calling cell is not involved in the cycle
+        raise raw.error
+      else:
+        raise objtypes.CellError(self.table_id, self.col_id, row_id, raw.error)
     if self.type_obj.is_right_type(raw):
       return self._make_rich_value(raw)
     return usertypes.AltText(str(raw), self.type_obj.typename())