archive: Introduce storageDiffError event

Signed-off-by: Alexandru Vasile <[email protected]>

Author: lexnv

src/api/archive_unstable_storageDiff.md

@@ -49,8 +49,6 @@ Where `subscription` is the value returned by this function, and `result` can be
 
 ### storageDiff
 
-The JSON object returned by this function has the following format:
-
 ```json
 {
     "event": "storageDiff",
@@ -79,8 +77,6 @@ The `storageDiff` event is generated for each storage difference between the two
 
 ### storageDiffDone
 
-The JSON object returned by this function has the following format:
-
 ```json
 {
     "event": "storageDiffDone",
@@ -89,6 +85,21 @@ The JSON object returned by this function has the following format:
 
 This event is always generated after all `storageDiff` events have been generated.
 
+No more events will be generated after a `storageDiffDone` event.
+
+### storageDiffError
+
+```json
+{
+    "event": "storageDiffError",
+    "error": "...",
+}
+```
+
+`error` is a human-readable error message indicating why the call has failed. This string isn't meant to be shown to end users, but is for developers to understand the problem.
+
+No more events will be generated after a `storageDiffError` event.
+
 ## Overview
 
 This function calculates the storage difference between two blocks. The storage difference is calculated by comparing the storage of the `previousHash` block with the storage of the `hash` block. If the `previousHash` parameter is not provided, the storage difference is calculated between the parent of the `hash` block and the `hash` block.