Quieter `Stop` handling on ctx result capture

In the `drain_to_final_msg()` impl, since a stream terminating
gracefully requires this msg, there's really no reason to `log.cancel()`
about it; go `.runtime()` level instead since we're trying de-noise
under "normal operation".

Also,
- passthrough `hide_tb` to taskc-handler's `ctx.maybe_raise()` call.
- raise `MessagingError` for the `MsgType` unmatched `case _:`.
- detail the doc string motivation a little more.

tractor/msg/_ops.py

@@ -374,7 +374,7 @@ class PldRx(Struct):
 
             case _:
                 src_err = InternalError(
-                    'Unknown IPC msg ??\n\n'
+                    'Invalid IPC msg ??\n\n'
                     f'{msg}\n'
                 )
 
@@ -499,7 +499,7 @@ async def maybe_limit_plds(
         yield None
         return
 
-    # sanity on scoping
+    # sanity check on IPC scoping
     curr_ctx: Context = current_ipc_ctx()
     assert ctx is curr_ctx
 
@@ -510,6 +510,8 @@ async def maybe_limit_plds(
     ) as msgdec:
         yield msgdec
 
+    # when the applied spec is unwound/removed, the same IPC-ctx
+    # should still be in scope.
     curr_ctx: Context = current_ipc_ctx()
     assert ctx is curr_ctx
 
@@ -525,16 +527,26 @@ async def drain_to_final_msg(
     list[MsgType]
 ]:
     '''
-    Drain IPC msgs delivered to the underlying IPC primitive's
-    rx-mem-chan (eg. `Context._rx_chan`) from the runtime in
-    search for a final result or error.
+    Drain IPC msgs delivered to the underlying IPC context's
+    rx-mem-chan (i.e. from `Context._rx_chan`) in search for a final
+    `Return` or `Error` msg.
 
-    The motivation here is to ideally capture errors during ctxc
-    conditions where a canc-request/or local error is sent but the
-    local task also excepts and enters the
-    `Portal.open_context().__aexit__()` block wherein we prefer to
-    capture and raise any remote error or ctxc-ack as part of the
-    `ctx.result()` cleanup and teardown sequence.
+    Deliver the `Return` + preceding drained msgs (`list[MsgType]`)
+    as a pair unless an `Error` is found, in which unpack and raise
+    it.
+
+    The motivation here is to always capture any remote error relayed
+    by the remote peer task during a ctxc condition.
+
+    For eg. a ctxc-request may be sent to the peer as part of the
+    local task's (request for) cancellation but then that same task
+    **also errors** before executing the teardown in the
+    `Portal.open_context().__aexit__()` block. In such error-on-exit
+    cases we want to always capture and raise any delivered remote
+    error (like an expected ctxc-ACK) as part of the final
+    `ctx.wait_for_result()` teardown sequence such that the
+    `Context.outcome` related state always reflect what transpired
+    even after ctx closure and the `.open_context()` block exit.
 
     '''
     __tracebackhide__: bool = hide_tb
@@ -572,7 +584,6 @@ async def drain_to_final_msg(
             # |_from tractor.devx._debug import pause
             #   await pause()
 
-
         # NOTE: we get here if the far end was
         # `ContextCancelled` in 2 cases:
         # 1. we requested the cancellation and thus
@@ -580,13 +591,13 @@ async def drain_to_final_msg(
         # 2. WE DID NOT REQUEST that cancel and thus
         #    SHOULD RAISE HERE!
         except trio.Cancelled as taskc:
-
             # CASE 2: mask the local cancelled-error(s)
             # only when we are sure the remote error is
             # the source cause of this local task's
             # cancellation.
             ctx.maybe_raise(
-                # TODO: when use this/
+                hide_tb=hide_tb,
+                # TODO: when use this?
                 # from_src_exc=taskc,
             )
 
@@ -659,7 +670,7 @@ async def drain_to_final_msg(
             # Stop()
             case Stop():
                 pre_result_drained.append(msg)
-                log.cancel(
+                log.runtime(  # normal/expected shutdown transaction
                     'Remote stream terminated due to "stop" msg:\n\n'
                     f'{pretty_struct.pformat(msg)}\n'
                 )
@@ -719,13 +730,19 @@ async def drain_to_final_msg(
                 pre_result_drained.append(msg)
                 # It's definitely an internal error if any other
                 # msg type without a`'cid'` field arrives here!
+                report: str = (
+                    f'Invalid or unknown msg type {type(msg)!r}!?\n'
+                )
                 if not msg.cid:
-                    raise InternalError(
-                        'Unexpected cid-missing msg?\n\n'
-                        f'{msg}\n'
+                    report += (
+                        '\nWhich also has no `.cid` field?\n'
                     )
 
-                raise RuntimeError('Unknown msg type: {msg}')
+                raise MessagingError(
+                    report
+                    +
+                    f'\n{msg}\n'
+                )
 
     else:
         log.cancel(