Always log the temp directory used by the DB migrator and how to override it on error

The DB migrator uses a temporary directory to export the OrientDB databases.

NEXUS-29410 added a message explaining where the tmp dir might be if there was not enough size there.

Some documentation was added mentioning that the migrator will check if there is at least 10GB free in the tmp dir.

There are several important useful things missing still:

1. The tmp dir location is never logged, unless there is an error
2. It is not documented how to change the temp dir used by the migrator
3. The error message displayed when there is not enough free space is not informative on where the temp dir came from or how to override it.

Expected

Correct these shortcomings.