Handle osuser+ldap and osuser+jira gracefully

The lack of support for migration from osuser (CONF-22046 and CONF-22061 are marked 'wont fix') is going to hurt customers for years to come. This will become a scenario that will occur for any pre-3.5 upgrade to post-3.5 upgrade, which means for the next few years it'll continue to show up.

It's therefore critical that we do as much as possible to handle these conditions gracefully. In addition to the initial effort at (CONF-22103) which does a validation step and logs the error, we should create error or warn level logs that are distinct and can be captured by regular expressions so Hercules can display messages. Furthermore, we should display links to these KB articles in the user interface, when one of these exceptions occurs.

As currently implemented in 3.5.1, logging will look like this for both OSUser with LDAP and osuser with JIRA:

com.atlassian.confluence.upgrade.UpgradeException: OSUser to Embedded Crowd pre upgrade validation failed.

The request here is to:

1. Change the logging so OSUser+LDAP is distinct from OSUser+JIRA
2. Change the regular expressions in the Knowledge base articles so it matches the distinct logging, so Hercules and the Support Tools Plugin can match the accurate message
3. After the validation check, display a link in the user interface to the appropriate knowledge base article (remember to use the tiny url, in case the name changes!).