Python expression preview doesn't always report errors

[tfmorris]

To Reproduce

1. Select "Add Column based on this Column"
2. With the expression cells.a.value.toNumber() - cells.b.value.toNumber(), switch the expression language to Python

Current Results

No error message is displayed, presumably because the response has an "ok" status:

{"code":"ok","results":[{"message":"Traceback (most recent call last):\n  File \"<string>\", line 2, in __temp_594255022__\nAttributeError: 'unicode' object has no attribute 'toNumber'\n"}]}

Expected Behavior

The error message should be displayed to the user

[tfmorris]

I don't think this is the correct fix. Errors are normally reported in red in the top pane of the dialog (where both screenshots says "No syntax error" in the related PR #4991). This change removes the line number (which is useful information for helping to figure out where in the Python program the error is), but the returned string is still being considered a value for the preview, not an error.

I haven't looked at it since, but when I wrote up the bug report, I clearly thought that the problem was with the "code": "ok" part of the response. That could be incorrect, but it's definitely worth looking at.

[thadguidry]

Could we also add a Cypress test for it?

[Sudo-Aryan]

can I try with this?