* Replace assert and NGRAPH_ASSERT with NGRAPH_CHECK; deprecate old classes

* Rename node_validation_assertion_string to something slightly more descriptive

* Fix syntax error in GPU BE

* Fix some messed-up includes

* More comments, more tests

* Fix reference to properly include check.hpp (should unbreak codegen)

* Add check.hpp include to gpu_util.cpp

* Comment updates

* Comment update

* Oops

* PR feedback: better formatting when context info or explanation are missing