FormsReach API errors

The submit API returns JSON problem envelopes. Each error.type is a stable URL on this host. Agents should parse error.code and show error.detail plus the hint on these pages.

• missing_api_key - 422 Unprocessable Entity
• form_not_found - 404 Not Found
• domain_denied - 403 Forbidden
• validation_error - 422 Unprocessable Entity
• payload_too_large - 413 Payload Too Large
• unsupported_media_type - 415 Unsupported Media Type
• insufficient_credits - 402 Payment Required
• rate_limited - 429 Too Many Requests
• not_found - 404 Not Found