Point users to upgrade documentation if they try to connect to db with existing jira data.

In the initial JIRA start we don't allow users to point to a db with existing data when configuring connection.
Currently we show an error, we need to modify the error message and add a link to upgrade documentation. Something like "The database specified is not empty. Please specify an empty database for your JIRA installation. If you want to upgrade the existing installation please follow this instructions." where 'this instructions' is a link to a documentation page.

Workaround

1. Stop JIRA.
2. Manually alter the $JIRA_HOME/dbconfig.xml or use the JIRA Configuration Tool to point to the database.
   Please ensure to follow the appropriate documentation for Connecting JIRA to a Database.
3. Start JIRA.