Let `pack_error()` take a msg injected `cid: str|None`

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):
+class StreamOverrun(
-    "This stream was overrun by sender"
+    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):
@@ -152,13 +163,15 @@ class MessagingError(Exception):
 
 def pack_error(
     exc: BaseException,
-    tb: str | None = None,
+    tb: str|None = None,
+    cid: str|None = None,
 
 ) -> dict[str, dict]:
     '''
-    Create an "error message" encoded for wire transport via an IPC
+    Create an "error message" which boxes a locally caught
-    `Channel`; expected to be unpacked on the receiver side using
+    exception's meta-data and encodes it for wire transport via an
-    `unpack_error()` below.
+    IPC `Channel`; expected to be unpacked (and thus unboxed) on
+    the receiver side using `unpack_error()` below.
 
     '''
     if tb:
@@ -175,19 +188,33 @@ 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}
+
+    pkt: dict = {'error': error_msg}
+    if cid:
+        pkt['cid'] = cid
+
+    return pkt
 
 
 def unpack_error(
 
     msg: dict[str, Any],
     chan=None,
-    err_type=RemoteActorError
+    err_type=RemoteActorError,
+    hide_tb: bool = True,
 
-) -> None | Exception:
+) -> None|Exception:
     '''
     Unpack an 'error' message from the wire
     into a local `RemoteActorError` (subtype).
@@ -196,7 +223,7 @@ def unpack_error(
     which is the responsibilitiy of the caller.
 
     '''
-    __tracebackhide__: bool = True
+    __tracebackhide__: bool = hide_tb
 
     error_dict: dict[str, dict] | None
     if (
@@ -309,6 +336,11 @@ def _raise_from_no_key_in_msg(
         # value out of the underlying feed mem chan!
         stream._eoc: bool = True
 
+        # TODO: if the a local task is already blocking on
+        # a `Context.result()` and thus a `.receive()` on the
+        # rx-chan, we close the chan and set state ensuring that
+        # an eoc is raised!
+
         # # when the send is closed we assume the stream has
         # # terminated and signal this local iterator to stop
         # await stream.aclose()
@@ -317,8 +349,8 @@ def _raise_from_no_key_in_msg(
         # raise a ``StopAsyncIteration`` **and** in our catch
         # block below it will trigger ``.aclose()``.
         raise trio.EndOfChannel(
-                'Context[{cid}] stream ended due to msg:\n'
+            f'Context stream ended due to msg:\n'
-                f'{pformat(msg)}'
+            f'{pformat(msg)}'
         ) from src_err
 
 
@@ -333,8 +365,7 @@ def _raise_from_no_key_in_msg(
     # is activated above.
     _type: str = 'Stream' if stream else 'Context'
     raise MessagingError(
-        f'{_type} was expecting a `{expect_key}` message'
+        f"{_type} was expecting a '{expect_key}' message"
-        ' BUT received a non-`error` msg:\n'
+        " BUT received a non-error msg:\n"
-        f'cid: {cid}\n'
+        f'{pformat(msg)}'
-        '{pformat(msg)}'
     ) from src_err