Migration may fail if an attachment is missing property content

Right now, if there is a Jira attachment attached to a step which is missing property content, the migration will fail since Xray expects it to be there.

Additionally, there are currently no logs for this, making it impossible to identify the issue(s) that caused the migration to fail.

The migration should not fail if this error occurs, and the issue id(s) of the affected issues should be logged.