Add: store the reason why db failed

[eight04]

Fixes #546

It saves the validation error so we can inspect it with

chromeLocal.getValue('dbInChromeStorageReason')

However, I don't know how to test this branch. Is there a way to make indexed DB fail manually?

[Mottie]

Should we also open a modal showing the error?

[eight04]

In the manager? I think it won't work if the user doesn't open the manager page.

[Mottie]

What about an alert then?

[eight04]

You can't use alert in the background page in Firefox.

[Mottie]

I'm not sure how to test this PR. What if we also store the URL (or just the domain; to ensure privacy) causing the error so we know the source? I thought I saw an error pop up when the Firefox add-ons manager page was open, but now I can't reproduce it.

[eight04]

The validation only happens when the DB is initializing i.e. when the extension startups, so there is no URL that could be recorded. I also don't think the error would relate to a specific page because db.js is used in the background page.

I'm not sure how to test this PR.

Then we just merge it?