[ES6 modules] Minor fixes to v8::ScriptModule

ScriptModule::Evaluate
- Make it a const method.
- Clarify a TODO that we don't currently have a codepath which use result of EvaluateModule.
-- Spec "run a module script" Step 7 currently only cares about exceptions thrown, and not about return value from successful completion.
-- https://html.spec.whatwg.org/multipage/webappapis.html#run-a-module-script

ScriptModule::Instantiate
- Call v8::TryCatch::SetVerbose(true), so that the instantiation errors will be reported to devtools console.

BUG=594639

Review-Url: https://codereview.chromium.org/2821903002
Cr-Commit-Position: refs/heads/master@{#464911}
2 files changed