- Add optional error_type parameter to respJsonError template for reliable
  client-side error detection
- Add httpCode parameter to return proper HTTP status codes
- Use HTTP 404 for not_found errors (tweet/user not found)
- Use "not_found" error_type for tweet and user not found errors

This allows clients to:
1. Check error_type === 'not_found' instead of parsing error messages
2. Rely on HTTP status codes for proper REST semantics

Example error response:
HTTP/1.1 404 Not Found
{
  "code": -1,
  "error": "Tweet not found",
  "error_type": "not_found"
}