ENSRainbow: Refine handling of invalid ENV vars

[lightwalker-eth]

Creating this story while reviewing PR 101.

Suggest setting the ENV vars used by ENSRainbow to invalid values. Ex: directories or files that don't exist, or to invalid port values (ex: negative integers, etc.. please see definition of parseInt). Please also consider invalid INPUT_FILE values.

Goal: Ensure that in all cases the error messages are useful and would help someone to fix the incorrectly set ENV var.

I see that we already have some logic added for this inside apps/ensrainbow/src/index.ts, however:

1. Shouldn't we use similar logic in apps/ensrainbow/src/ingest.ts (and anywhere else relevant)?
2. Shouldn't we move this logic such that it is defined in only a single file and imported as needed?

Also suggest writing utility functions that wrap reading values out of ENV vars. Please see utility functions in the following file for some inspirations: https://github.com/namehash/ensnode/blob/main/apps/ensnode/src/lib/ponder-helpers.ts