[No QA] Add documentation about error onyx best practice under API.md

docs/API.md

@@ -42,6 +42,19 @@ The data will automatically be sent to the user via Pusher.
 
 #### WRITE Response Errors
 When there is an error on a WRITE response (`jsonCode!==200`), the error must come back to the client on the HTTPS response. The error is only relevant to the client that made the request and it wouldn't make sense to send it out to all connected clients.
+
+Error messages should be returned and stored as a String under the `error` property. If absolutely needed, additional error properties can be stored under other, more specific fields that sit at the same level as `error`:
+```php
+[
+    'onyxMethod' => Onyx::METHOD_MERGE,
+    'key' => OnyxKeys::WALLET_ADDITIONAL_DETAILS,
+    'value' => [
+        'error' => 'We\'re having trouble verifying your SSN. Please enter the full 9 digits of your SSN.',
+        'errorCode' => 'ssnError'
+    ],
+]
+```
+
 ### Hybrid READ/WRITE Responses
 There are some commands which might technically be a READ and a WRITE.
 
@@ -68,4 +81,4 @@ For example: Accessing the data for a chat report will return the data if the re
 
 ### How should we remove local data from the store if the data no longer exist in the server? eg. local policy data for policies the user is no longer a member of.
 
-Use `Onyx.set(key, null)` for each data (eg. policy) not found in the server.
+Use `Onyx.set(key, null)` for each data (eg. policy) not found in the server.