feat(tvix/cli): add errors module with drv construction errors

These will be threaded through to eval through the new `TvixError`
variant.

Change-Id: Ia0d3f8710dcf26bb95015cd2a6a2b2911f06343f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7842
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/cli/src/main.rs

@@ -1,3 +1,4 @@
+mod errors;
 mod known_paths;
 mod nix_compat;
 mod refscan;