Add `StreamOverrun.sender: tuple` for better handling

Since it's generally useful to know who is the cause of an overrun (say
bc you want your system to then adjust the writer side to slow tf down)
might as well pack an extra `.sender: tuple[str, str]` actor uid field
which can be relayed through `RemoteActorError` boxing. Add an extra
case for the exc-type to `unpack_error()` to match B)

tractor/_exceptions.py

@@ -134,8 +134,19 @@ class NoRuntime(RuntimeError):
     "The root actor has not been initialized yet"
 
 
-class StreamOverrun(trio.TooSlowError):
-    "This stream was overrun by sender"
+class StreamOverrun(
+    RemoteActorError,
+    trio.TooSlowError,
+):
+    '''
+    This stream was overrun by sender
+
+    '''
+    @property
+    def sender(self) -> tuple[str, str] | None:
+        value = self.msgdata.get('sender')
+        if value:
+            return tuple(value)
 
 
 class AsyncioCancelled(Exception):
@@ -175,7 +186,15 @@ def pack_error(
         'src_actor_uid': current_actor().uid,
     }
 
-    if isinstance(exc, ContextCancelled):
+    # TODO: ?just wholesale proxy `.msgdata: dict`?
+    # XXX WARNING, when i swapped these ctx-semantics
+    # tests started hanging..???!!!???
+    # if msgdata := exc.getattr('msgdata', {}):
+    #     error_msg.update(msgdata)
+    if (
+        isinstance(exc, ContextCancelled)
+        or isinstance(exc, StreamOverrun)
+    ):
         error_msg.update(exc.msgdata)
 
     return {'error': error_msg}