Add a missing check for exceptions thrown in methods implemented in private scripts

method.cpp misses a check for a returned value of a method implemented in a private script.
This CL adds the check. Other places already have the check.

(This call path is not yet used in production.)

BUG=341031

Review URL: https://codereview.chromium.org/462623002

git-svn-id: svn://svn.chromium.org/blink/trunk@179943 bbb929c8-8fbe-4397-9dbb-9b2b20218538