[Cgl] cgl, cpx, mir, error

[Lou Hafer]

Ted, Matt,

	I know I must sound like a perpetual fussbudget, but ...  it's not that
simple.  For one thing, how will you know there's an error?  Look for a bogus
return value?  What's bogus?  Look for a throw?  Hah!  That's more inconsistent
than most across implementations.  Who's going to check?  Only the client can
really say whether they're looking at a bogus return value --- that'll depend on
the state of the OSI (unloaded, imposed solution, solution from solver).

	It's a reasonable idea to provide a default (if inefficient)
implementation in the base class.  But arguably better to fix OsiCpx so that it
knows when to use a solution supplied by the user and when to use a solution
from the solver, and when it has no solution available.

							Lou