Add error handling to test mock servers

[lalitcap23]

Problem

Test helper HTTP servers were crashing with confusing 15+ line stack traces when they failed to start (e.g., port already in use). This made debugging test failures unnecessarily difficult.

Changes

Added error event handlers to server.listen() in all 5 test helper servers:

• packages/w3up-client/test/helpers/bucket-server.js
• packages/w3up-client/test/helpers/receipts-server.js
• packages/w3up-client/test/helpers/gateway-server.js
• packages/upload-client/test/helpers/bucket-server.js
• packages/upload-client/test/helpers/receipts-server.js

Before & After

Before:

throw er; // Unhandled 'error' event
^
Error: listen EADDRINUSE: address already in use :::9999
at Server.setupListenHandle [as _listen2] (node:net:1908:16)
[... 10+ more lines ...]

After:
Failed to start server on port 9999: listen EADDRINUSE: address already in use :::9999

Testing

Manually tested by starting duplicate servers on the same ports. All 5 servers now display clear error messages and exit with proper error codes.

Benefits

• ✅ Clear, one-line error messages
• ✅ Easier debugging of port conflicts
• ✅ Proper exit codes (1 for errors)
• ✅ Follows Node.js best practices

Fixes #554

[lalitcap23]

@travis , could you please review this PR?

[lalitcap23]

@travis , do i need to change anything else .

[travis]

it looks like you have some linting issues - please take a look at the failing CI check below

[lalitcap23]

@travis can u review it .

[lalitcap23]

@travis , can u review it and suggest the changes .