BulkImports: Add custom ErrorTracker to log to the `imports.log` as well
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### Proposal
Currently BulkImports is using `Gitlab::ErrorTracker` to track exceptions. This is great, but it only logs to the `exceptions.log` file. Since most of the import process is logged within the `imports.log`, I propose to create a `Gitlab::Import::ErrorTracker` that log to both `exceptions.log` and `imports.log`. This will make debugging and following a import process easier.
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