Improve error checking and reporting

[rjplevin]

I'm trying to debug a problem running queries against the XMLDB, but the error messages are not as helpful as they could be:

• The XMLDB.openDatabase methods throw an error if the DB was previously open, but not if the call to new XMLDB() fails.
• DbViewer.runBatch doesn't check that the database was opened correctly either. This results in later attempts to use the not-open DB, producing the misleading error message "Could not find scenarios to run."
• DbViewer.runBatch calls XMLDB.getInstance to see if the DB is open, but this produces the spurious message "The database is not open". It would be better to have a method that just returns whether XMLDB.xmldbInstance == NULL for this use case.