Add `NRSC5DUI_DATA` environment variable

[bleonard252]

This PR adds an environment variable intended for use when nrsc5-dui is installed somewhere immutable. A few fixes for assumptions about the runtime environment were applied. I also had it print the actual error when it failed to write the config file (as this was an issue I was having).

When NRSC5DUI_DATA is set to a writeable, empty directory:

• nrsc5-dui creates the cfg, aas, and map directories
• Those directories are not based on runtimeDir but instead based on the directory given

When NRSC5DUI_DATA is not set:

• nrsc5-dui assumes its value is the runtimeDir and proceeds as normal

It may be beneficial to apply this as a "squash" merge.

[markjfine]

Good idea. One minor nit to fix and I'll accept/merge.

[markjfine]

Awesome. Thank you!

[markjfine]

As simple and elegant as this is, it's given me some ideas on how to finally solve the issue of being able to easily swap configurations (mostly the bookmarks and associated station logos) when using the app in more than one radio market (e.g, DC and/or NYC).