feat(docs): document errors and errno

API.md

@@ -0,0 +1,58 @@
+# Firefox Accounts email service API
+
+## Overview
+
+### Response format
+
+```
+{
+  "code": 400,  // Matches the HTTP status code
+  "errno": 107, // Stable application-level error number
+  "error": "Bad Request", // String description of the error type
+  "message": "Invalid parameter in request body", // Specific error message
+}
+```
+
+### Defined errors
+
+- `code: 500, errno: 100`: Rocket Error
+- `code: 400, errno: 101`: Missing Email Parameters
+- `code: 500, errno: 102`: Invalid Email Parameters
+- `code: 500, errno: 103`: Invalid Provider
+- `code: 500, errno: 104`: Provider Error
+- `code: 500, errno: 105`: Email Parsing Error
+- `code: 429, errno: 106`: Bounce Complaint Error
+- `code: 429, errno: 107`: Bounce Soft Error
+- `code: 429, errno: 108`: Bounce Hard Error
+- `code: 500, errno: 109`: Database Error
+- `code: 500, errno: 110`: Queue Error
+- `code: 500, errno: 111`: Invalid Notification Type
+- `code: 500, errno: 112`: Missing Notification Payload
+- `code: 500, errno: 113`: Missing SQS Message Field
+- `code: 500, errno: 114`: SQS Message Hash Mismatch
+- `code: 500, errno: 115`: SQS Message Parsing Error
+- `code: 500, errno: 116`: Duration Error
+- `code: 500, errno: 117`: MessageDataError
+- `code: 500, errno: 118`: Not Implemeneted
+
+The following errors include additional response properties:
+
+- `errno: 100`
+- `errno: 101`
+- `errno: 102`
+- `errno: 103`
+- `errno: 104` name
+- `errno: 105`
+- `errno: 106` address, bouncedAt, bouce
+- `errno: 107` address, bouncedAt, bouce
+- `errno: 108` address, bouncedAt, bouce
+- `errno: 109`
+- `errno: 110`
+- `errno: 111`
+- `errno: 112`
+- `errno: 113` queue, field
+- `errno: 114` queue, hash, body
+- `errno: 115` queue, message, body
+- `errno: 116`
+- `errno: 117`
+- `errno: 118`