chore(web): Error handling — 404 for invalid challenge, friendly messages for runner errors

[ArchILLtect]

Problem / context

Users need clear feedback without leaking stack traces.

Proposed solution

• Standardize outcome mapping and error messages in Practice and Drill flows
• Validate required params (language, code); handle missing values

In scope / Out of scope

In scope:

• Servlet/JSP message blocks; basic validation
  Out of scope:
• Centralized error pages

Acceptance criteria

• Invalid IDs → 404 in both modes
• Runner exceptions show friendly error panel; logs capture details

Area

area:web

Dependencies / related issues

Practice Mode, Drill Mode submit

Checklist

• I’ve checked existing issues for duplicates.
• I can help implement this and open a PR.