(core) Adding traceback to trigger formulas

Summary:
Traceback is available on the Creator Panel in the formula editor. It is evaluated the same way as for normal formulas.
In case when the traceback is not available, only the error name is displayed with information that traceback is not available.
Cell with an error, when edited, shows the previous valid value that was used before the error happened (or None for new rows).
Value is stored inside the RaisedException object that is stored in a cell.

Test Plan: Created tests

Reviewers: alexmojaki

Reviewed By: alexmojaki

Subscribers: alexmojaki, dsagal

Differential Revision: https://phab.getgrist.com/D3033

sandbox/grist/test_engine.py

@@ -12,6 +12,7 @@ import engine
 import logger
 import useractions
 import testutil
+import objtypes
 
 log = logger.Logger(__name__, logger.DEBUG)
 
@@ -262,6 +263,13 @@ class EngineTestCase(unittest.TestCase):
                                   for tbl in list_of_tables
                                   for col in tbl.columns))
 
+  def assertFormulaError(self, exc, type_, message, tracebackRegexp=None):
+    self.assertIsInstance(exc, objtypes.RaisedException)
+    self.assertIsInstance(exc.error, type_)
+    self.assertEqual(str(exc.error), message)
+    if tracebackRegexp:
+      self.assertRegex(exc.details, tracebackRegexp)
+
   def assertViews(self, list_of_views):
     """
     Verifies that the given View test-records correspond to the metadata for views/sections/fields.