lib: consolidate lazyErrmapGet()

There are currently two implementations of this function.
This commit removes the redundancy, and removes "lazy" from
the name.

PR-URL: https://github.com/nodejs/node/pull/29285


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>