[Extensions] Check |context_| in ModuleSystem::ExceptionHandler
[Extensions] Check |context_| in ModuleSystem::ExceptionHandler

In extensions unittests, the exception handler is subclassed and created
without an associated ScriptContext, but it then calls
CreateExceptionString(), which accessed context_. Add a check.

BUG=None
TEST=Throw an exception in some extensions binding code in a test.
     The test should instead of cras

Review URL: https://codereview.chromium.org/1878943010

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#387409}

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