Catch up to removal of ASSERT macro.

Commits 47bb5cd and c4c771e of opm-core refactored the exception
throwing code of opm/core/utility/ErrorMacros.hpp resulting in the
removal of the existing ASSERT macro and the introduction of its
functional replacement, OPM_ERROR_IF.  Use the latter in place of the
former.

Note: The ASSERT macro removal was included in PR OPM/opm-core#352.