Improve user-facing error messages for invalid tokens

[mroderick]

Context

PR #2529 (recently merged) fixed a production crash where invalid invitation tokens caused 500 errors (NoMethodError: undefined method 'member' for nil). The fix changed find_by to find_by! in before_action callbacks, so invalid tokens now return a proper 404 response.

The Problem

In the PR review, @till raised a valid UX concern:

"But doesn't that make for rather bad UX? Assumes whoever reads it, knows what token we mean"

Currently, users visiting /invitations/invalid-token see:

Page not found
If you entered a web address please check it was correct.
In the meantime why don't you join our community on Slack?

This message is generic and doesn't explain:

• What "token" means
• Why the link doesn't work
• What the user should do next

Proposed Solution

Create a custom exception (CodebarResourceNotFound) that carries a user-friendly message, plus a new error template that displays context-specific text.

Example messages by resource type:

• Invitations: "This invitation link is invalid or has expired. Please check your email for the correct link or contact the organisers."
• Events: "This event page doesn't exist or has been removed."
• Workshops: "We couldn't find the workshop you're looking for."

Implementation Approach

1. Create lib/codebar_errors.rb with CodebarResourceNotFound exception class
2. Add an exception handler in ApplicationController for this custom exception
3. Create a new error template (errors/not_found_user_friendly.html.haml) showing the custom message
4. Update WorkshopInvitationConcerns#set_invitation to raise the custom exception instead of letting Rails return generic 404
5. Phase 1: Invitations only (controllers using WorkshopInvitationConcerns)

This is a low-risk improvement that can be extended to other resource types (events, meetings, feedback requests) in future phases.

Questions for Discussion

1. Is this the right UX approach, or should we consider redirects with flash messages instead?
2. Should we start with invitations only, or include all resource types from fix: use find_by! to return 404 for invalid tokens #2529?
3. Any other edge cases to consider for the error message?