collect errors on bulk operations

[Ptosiek]

Problem

Errors during bulk operations are not returned to the user in dry_run mode. Instead the exception is caught and logged and user can proceed to real import (which fails).

An extra savepoint is also used to rollback the transaction on dry_run/errors, but we can used the current transaction block instead to achieve the same.

Solution

Make errors raised during bulk operation part of result.base_errors so they can be returned to the user.

Remove extra savepoint.

Acceptance Criteria

Tests were written + documentation updated.

[coveralls]

Coverage: 100.0%. Remained the same when pulling 07b57f7 on Ptosiek:raise-errors-on-bulk into 1e5eb96 on django-import-export:main.

[matthewhegarty]

Looks good thanks - I suggest a couple of minor changes for backwards compatibility and style.