(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/functions/info.py

@@ -12,6 +12,7 @@ import six
 import column
 from functions import date      # pylint: disable=import-error
 from functions.unimplemented import unimplemented
+from objtypes import CellError
 from usertypes import AltText   # pylint: disable=import-error
 from records import Record, RecordSet
 
@@ -597,6 +598,9 @@ def _prepare_record_dict(record, dates_as_iso=False, expand_refs=0):
       result[col_id] = val
     except Exception as e:
       result[col_id] = None
+      while isinstance(e, CellError):
+        # The extra information from CellError is redundant here
+        e = e.error  # pylint: disable=no-member
       errors[col_id] = "%s: %s" % (type(e).__name__, str(e))
 
   if errors: