Expose BulkImports::Failure#exception_message in the API

What does this MR do and why?

This MR updates BulkImports API in order to expose exception_message and provide a bit of clarity in what went wrong with the migration. Exposed attribute gets:

• Any files paths looking like /foo/bar get filtered and replaced with [FILTERED]
• Truncated to 72 characters (even though original db value is 255 characters). This is mainly to cover a situation if a message contains sensitive information we truncate it even further. 72 characters is a guess based on standard commit title length, can be adjusted if needed

Mentions #363339 (closed)

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