throw IOException when failing to load svm_model

[StaffanArvidsson]

svm.svm_load_model returned null when given a non valid file, make more sense to throw an exception instead (now throws IOException).
Added svm_print_err_string that works the same way as svm_print_string in svm.java. Now there's a way to remove printing errors directly to System.err which is bad if you wish to use libsvm as part of another java project. Added setter method for setting the svm_print_err_string