Add nonlinear program warning

Previously in `NonlinearProgram`, if some decision variables are bounded by equality bounding box constraint, then we take those decision variables out of the program for faster speed. This works well, except sometimes a constraint has all decision variables being taken out, and this would cause SNOPT to throw INFO 11, since the constraint becomes a constant, and usually this constant does not match the bounds exactly. This is hard to debug. Now we throw warnings to help debugging.
Sorry to nit-pick, but can we correct the english in this (and the following lines) to something like:

The decision variables in nonlinear inequality constraint %d are all fixed (due to equality bounding box constraints on the decision variables).  Consider either removing this constraint, or relaxing the bounds on the decision variables.