refactor(tvix/cli): describe errors with thiserror

This is much less code, and makes it much easier to read.

Change-Id: I9028f226105f905c2cc2cabd33907ff493e26225
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7938
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>

tvix/cli/Cargo.toml

@@ -18,6 +18,7 @@ smol_str = "0.1"
 aho-corasick = "0.7"
 ssri = "7.0.0"
 data-encoding = "2.3.3"
+thiserror = "1.0.38"
 
 [dev-dependencies]
 test-case = "2.2.2"