Enhance the error handling in SELECT():

Enhance the error handling in SELECT():

1. If the core class is not defined in the IDL, issue a
helpful message before bailing out.

2. If the core class is virtual, issua a helpful message
and bail out, instead of building a doomed query.

3. If a class referenced in the SELECT clause is not
included in the FROM clause, issue a helpful message and
bail out.  (The old code silently ignored it, and then
built a query that bombed out due to an extra comma.)

git-svn-id: svn://svn.open-ils.org/ILS/trunk@12279 dcc99617-32d9-48b4-a31d-7c20da2025e4