[ES6 modules] ScriptModule::compile() exceptions are handled via isolate's MessageListener

This CL removes a TODO comment and clarify that ScriptModule::compile errors are
routed via isolate's MessageListener.

Bonus: ScriptModule ctor w/o args is used to create null ScriptModule.

BUG=594639

Review-Url: https://codereview.chromium.org/2798533002
Cr-Commit-Position: refs/heads/master@{#461684}
2 files changed