Fix misleading "unknown endpoint" errors on stealth operations

[0xAJPanda]

Hey team! Been testing the webcli on devnet (love what you're building btw) and ran into something that had me chasing my tail for a while.

When stealth send hits an error — like the recipient not having a registered view pubkey — the response comes back as "unknown endpoint: POST /api/stealth/send" instead of the actual error. The route exists and works fine, but the error handler at line 291 catches any API response with an empty body and slaps "unknown endpoint" on it, even when it's a 500 from a real route.

Took me a while to figure out the actual errors were hiding in the EXCEPTION_WHAT response header, which most people probably wouldn't think to check.

What this changes:

• 404s still show "unknown endpoint" (as expected)
• 500s now show the actual status code + the exception detail from EXCEPTION_WHAT when available

So instead of:

{"error": "unknown endpoint: POST /api/stealth/send"}

You'd get:

{"error": "internal error on: POST /api/stealth/send (status 500)", "detail": "type must be string, but is null"}

Small change (10 lines), shouldn't break anything. Happy to adjust if you'd prefer a different format for the error response.

Keep up the great work on the webcli — the encrypt/decrypt flow works really well.