Elixir: Add `System` function that raises if ENV var is not present

[mwean]

Continuing from this forum discussion: https://groups.google.com/forum/#!msg/elixir-lang-core/yw54VGwGkt4/GGoh7ZEmFAAJ

The current method of getting ENV variables is System.get_env, but this returns nil if the variable is missing. This can lead to confusing errors if not explicitly handled, so it would be nice to have fetch_env and fetch_env! functions that would return an error tuple or raise on missing variables. This will fail fast and make it easier to debug.

[josevalim]

Please see rationale for closing this on #5608.

[joeellis]

Was also looking for this but noticed the posted rationale link just goes to this page. I assume @josevalim meant to post the PR where it was discussed, you can find that here: https://github.com/elixir-lang/elixir/pull/5610#issuecomment-270127996