[REF] iap: adapt include on CrashManager

In iap, we want to handle InsufficientCreditErrors in a special way
(present the user with a prompt to buy more credits) instead of the
usual error handling that shows a crash dialog. Previously, this was
done by doing an include on the crash manager. In wowl, we no longer use
the crash manager in the backend, and replaced it with an error service
that allows developpers to register error handlers for these kinds of
use cases.

This commit adapts what was previously an include on the crash manager
by instead adding a handler to the error handler registry, the file has
also been renamed accordingly.

The file iap_credit.js has also been removed, as it adds an action to
the action registry that is no longer called by anything, and is dead
code.