reduce precompile() failure severity to a warning (#39905)
Many users (including Base) are calling `@assert`, despite that this is
not what assert should be used to mark, for many reasons.
This happened to also reveal a small number of errors, so also detect
those (for fixing later).
Refs: https://github.com/JuliaLang/julia/commit/c0f9666d0b94b213c7ff9e64a7b4e5268aa0e18b#commitcomment-47782674
