(core) Error explanations from friendly-traceback

Summary: Extend formula error messages with explanations from https://github.com/friendly-traceback/friendly-traceback. Only for Python 3.

Test Plan: Updated several Python tests. In general, these require separate branches for Python 2 and 3.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3542

sandbox/grist/test_engine.py

@@ -270,7 +270,7 @@ class EngineTestCase(unittest.TestCase):
   def assertFormulaError(self, exc, type_, message, tracebackRegexp=None):
     self.assertIsInstance(exc, objtypes.RaisedException)
     self.assertIsInstance(exc.error, type_)
-    self.assertEqual(str(exc.error), message)
+    self.assertEqual(exc._message, message)
     if tracebackRegexp:
       self.assertRegex(exc.details, tracebackRegexp)