TrustRegionSolver: Make detection of trust-region underflows work

There already was code that printed a warning when the trust-region
radius dropped below the required solver tolerance.  (If that happens,
the solver terminates, because the small trust-region forces the
correction to be smaller than the required tolerance.  However,
the code didn't warn the user about this, and such cases almost
looked like successful runs.)

Unfortunately, the warning code didn't work, because the standard
solver termination criterion stopped the TR iteration before
reaching the check for the trust-region size.