(core) Render unmarshallable values as non-errors, using their repr() strings.

Summary:
- Instead of sending an "UnmarshallableError" as an exception, introduce an
  "Unmarshallable" type of value, represented as ['U', repr(value)]
- Unmarshallable values are rendered using a bluish text color, no longer a
  pink background.
- Factor out ErrorDom to be simpler and cleaner.
- Add GristObjCode enum, and simplify related helpers.
- Use safe_repr() for when repr() itself fails
- Handle conversion errors using safe_repr() when str() fails

Test Plan: Added a test case based on a fixture covering a bunch of cases.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2584

app/common/ValueFormatter.ts

@@ -22,6 +22,7 @@ export function formatObject(args: [string, ...any[]]): string {
     case 'd': return moment.tz(objArgs[0] * 1000, 'UTC').format("YYYY-MM-DD");
     case 'R': return `${objArgs[0]}[${objArgs[1]}]`;
     case 'E': return gristTypes.formatError(args);
+    case 'U': return String(args[1]);
     case 'P': return PENDING_DATA_PLACEHOLDER;
   }
   return objType + "(" + JSON.stringify(objArgs).slice(1, -1) + ")";