Replace uses of assert for non-debug error checking

[manopapad]

It is common practice to not use assert for error checking, enough so that various toolchains (e.g. cmake, conda compiler activation scripts) set NDEBUG on release builds. Legate in particular has to meticulously remove these instances of NDEBUG to get its build of Realm functioning properly.

Realm might want to use a custom macro that's not affected by the value of NDEBUG, or keep the assertions but make sure they don't contain any business logic that is in danger of getting compiled out.