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.
Fixed. Thanks for correcting the English