adds support for building the model of a program for which the classpath is incomplete

> It could be great to report problems as warning or debug if we are in
> "noclasspath" mode.
>
when you compile with "noclasspath", there may be a large number of
compilation errors because of the missing classpath.

I don't see an automated way to distinguish between a "missing classpath
error" and another source code error.

If we display them all, that's a lot of logging information, and too
much easily means "useless"

--Martin