G-API: replace GAPI_Assert() with 'false' and '0' to GAPI_Error()
* gapi: GAPI_Error() macro
* gapi: replace GAPI_Assert() with 'false' and '0' to GAPI_Error()
* build: eliminate 'unreachable code' after CV_Error() (MSVC 2015)
* build: eliminate 'unreachable code' warning for MSVS 2015/2017
- observed in constructors stubs with throwing exception
* G-API: Make GFrame a new (distinct) G-type, not an alias to GMat
- The underlying host type is still cv::Mat, a new cv::MediaFrame
type is to be added as a separate PR
* Fix warnings and review comments
- Somewhow there was a switch() without a default: clause in Fluid
