failed upgrade to new user management due to missing osuser.xml

Support receives significant number of request for assistance from customers regarding failed upgrade to JIRA 4.3+.

Currently implemented logic of the upgrade task does not take into account the external password management flag. We should consider checking on this attribute and fail the upgrade if osuser.xml isn't found.

A meaningful error should be produced in UI and the log file asking users to supply osuser.xml in order to continue with the upgrade.

Since JIRA 4.4 release (last 2w) Support has 12 issues for it that took 260h to be resolved.