[pp] add 'error' event

docs/configuration.rst

@@ -6126,6 +6126,8 @@ Description
         After a file got moved to its target location
     ``skip``
         When skipping a file download
+    ``error``
+        After a file download failed
     ``post``
         When starting to download all files of a `post`,
         e.g. a Tweet on Twitter or a post on Patreon.

gallery_dl/job.py

@@ -347,6 +347,9 @@ class DownloadJob(Job):
                 self.status |= 4
                 self.log.error("Failed to download %s",
                                pathfmt.filename or url)
+                if "error" in hooks:
+                    for callback in hooks["error"]:
+                        callback(pathfmt)
                 return
 
         if not pathfmt.temppath: