add warnings if some rows of the gradient matrix is empty in NonlinearProgram
add documentation to solve function in NonlinearProgram, for info 11 and 12
fix a typo
fix the English
Fix the documentation for INFO 11 in NonlinearProgram, add more print out message if INFO 11 is returned
Merge pull request #616 from hongkai-dai/add-NonlinearProgram-warning

Add nonlinear program warning