(core) Fix another cause of inconsistency that can be triggered by bad DocActions.

Summary:
An incorrect DocAction (as possible from an Undo of a non-last action)
could cause RemoveRecord on an already missing record. This used to
create an invalid undo, and wreak havoc when a series of DocActions
later fails and needs to be reverted.

To fix, consider RemoveRecord of a missing record to be a no-op.

Test Plan: Includes a new test case that triggers the problem.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2717

sandbox/grist/engine.py

@@ -434,7 +434,8 @@ class Engine(object):
       collist = sorted(actions.transpose_bulk_action(meta_columns),
                        key=lambda c: (c.parentId, c.parentPos))
       raise AssertionError("Internal schema inconsistent; extra columns in metadata:\n"
-          + "\n".join('  ' + str(schema.SchemaColumn(c.colId, c.type, bool(c.isFormula), c.formula))
+          + "\n".join('  #%s %s' %
+                      (c.id, schema.SchemaColumn(c.colId, c.type, bool(c.isFormula), c.formula))
                       for c in collist if c.parentId not in valid_table_refs))
 
   def dump_state(self):