Improve IDB testing / mitigation

[bhollis]

May fix #9716.

This:

• Backports the fix from Improve error handling jakearchibald/idb-keyval#164 so we hopefully get a better error than null.
• Adds a get test to the StorageTest.
• Adds a second layer of "don't crash" to the places where we rely on IDB get.
• Restores the delete-database hack to StorageTest. My new hypothesis is that that was working, but we were filtering out the exception reporting to Sentry. I've removed our ignored errors list to make sure they get through (though I'll probably have to add some back in, I don't know what they were all for).