Some of the Firefox profile backup code is executed within a web worker, so errors thrown in the worker do not automatically maintain their full context when caught in the main process.
This change creates a BackupError for throwing errors with causes specific to Firefox profile backup. The new error type is configured to work with the PromiseWorker machinery in the Firefox codebase in order to serialize and deserialize error details across the worker boundary.
Differential Revision: https://phabricator.services.mozilla.com/D215920
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