Clarify documentation/schema about on_error hook running if there's an error in another hook (#202).

borgmatic/config/schema.yaml

@@ -345,9 +345,11 @@ map:
             on_error:
                 seq:
                     - type: scalar
-                desc: List of one or more shell commands or scripts to execute in case an exception has occurred.
+                desc: |
+                    List of one or more shell commands or scripts to execute when an exception occurs
+                    during a backup or when running a hook.
                 example:
-                    - echo "Error while creating a backup."
+                    - echo "Error while creating a backup or running a hook."
             umask:
                 type: scalar
                 desc: Umask used when executing hooks. Defaults to the umask that borgmatic is run with.

docs/how-to/add-preparation-and-cleanup-steps-to-backups.md

@@ -27,13 +27,13 @@ not if an error occurs in a previous hook or in the backups themselves.
 
 ## Error hooks
 
-borgmatic also runs `on_error` hooks if an error occurs. Here's an example
-configuration:
+borgmatic also runs `on_error` hooks if an error occurs, either when creating
+a backup or running another hook. Here's an example configuration:
 
 ```yaml
 hooks:
     on_error:
-        - echo "Error while creating a backup."
+        - echo "Error while creating a backup or running a hook."
 ```
 
 ## Hook output