Add data import failure pixels

[samsymons]

Task/Issue URL: https://app.asana.com/0/0/1202158855860055/f
Tech Design URL:
CC: @brindy

Description:

This PR adds a new pixel which reports the import source, import type, and error message when data import fails.

The format and parameters of this pixel are found here.

The pixel from this PR will be used to drill down into where the importer is failing and why next week, and translate into some fixes. One thing we're missing is the actual versions of the files we're reading from, which will be added in a follow-up later.

Steps to test this PR:

First: turn on pixel logging in Logging.swift.

Next: It's hard to set up the import failure conditions needed, so I recommend picking one of the import classes and adding a hardcoded failure to it. This won't help with testing whether the actual importer classes themselves are returning the right error at the right time, but I haven't modified those here and plan to expand their test suites in a PR to ensure that all cases are covered next week.

You can use ChromiumDataImporter.swift and add the following line to the top of the importData function (of course, please tweak the failure type to test that the pixel is accurate):

completion(.failure(.bookmarks(.browserNeedsToBeClosed)))
return

Testing checklist:

• Test with Release configuration
• Test proper deallocation of tabs
• Make sure committed submodule changes are desired

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Internal references:

Software Engineering Expectations
Technical Design Template
When ready for review, remember to post the PR in MM

[brindy]

Did a bit of a smoke test, checked importing still works, etc. Difficult to test the scenarios of course, but it LGTM. Just one comment about the logging strings to use, I don't have a strong opinion.