fix: cleanup migration error when retrying a failed import

Closes #636 (closed)

migration_error is zeroed once an import is sure to start (after shouldImport).

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Related to #633 (closed)

A negative testing delay will trigger an error in the preimport. This solution doesn't add new opaque options and it doesn't rely on timeouts, it can replicate a failed pre import easily and shouldn't cause flaky tests.

Note: it seems withMigrationSlowImport was only used with waitForever so there could be some usability/naming improvements.