[ES6 modules] Add bindings for module record's [[ErrorCompletion]] field's [[Value]].

This CL exposes v8::Module::GetException() as ScriptModule::ErrorCompletion().

This CL also updates ModulatorImpl::GetError(ModuleScript*) impl to match the latest "#concept-module-script-error" concept.
ModulatorImpl change will be covered by external/wpt/html/semantics/scripting-1/the-script-element/instantiation-error* once the entire module infra is updated.

Bug: 594639, 727299, https://github.com/whatwg/html/pull/2674 , https://github.com/tc39/ecma262/pull/916
Change-Id: I4d4847c2031f7456e659984b00542fdb08c68dd9
Reviewed-on: https://chromium-review.googlesource.com/554633
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#483521}
5 files changed