Replace panic("not reachable") with panic(NewUnreachableError())

[turbolent]

Description

Unimportant, tiny improvements:

• Replace every panic("not reachable") with panic(UnreachableError())

• In a smoke test, improve error when flag cannot be parsed by replacing:

  panic("Failed to parse seed flag (hex string)")`

  with

  fmt.Fprintf(os.Stderr, "failed to parse seed flag (hex string) %s: %s", flagSeedHex, err) 

---

• Targeted PR against main branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[fxamacker]

Nice! 👍