Updated examples

... in a way to really return error codes when an error happens. This
way we can easily use a new testing feature from the build system.
Small fix in the OCP class...

... so that it does not report an error when number of intervals in
the ctor is set to 0 -- this would return a warning when an NLP is
defined.
Merge branch 'master' of https://github.com/mvukov/acado