Catch `KeyError` on bcast errors which pop the sub

Not sure how i missed this (and left in handling of `list.remove()` and
it ever worked for that?) after the `samplerd` impl in 5ec1a72 but, this
adjusts the remove-broken-subscriber loop to catch the correct
`set.remove()` exception type on a missing (likely already removed)
subscription entry.

piker/data/_sampling.py

@@ -87,7 +87,6 @@ class Sampler:
     # holds all the ``tractor.Context`` remote subscriptions for
     # a particular sample period increment event: all subscribers are
     # notified on a step.
-    # subscribers: dict[int, list[tractor.MsgStream]] = {}
     subscribers: defaultdict[
         float,
         list[
@@ -240,8 +239,11 @@ class Sampler:
         subscribers for a given sample period.
 
         '''
+        pair: list[float, set]
         pair = self.subscribers[period_s]
 
+        last_ts: float
+        subs: set
         last_ts, subs = pair
 
         task = trio.lowlevel.current_task()
@@ -281,7 +283,7 @@ class Sampler:
         for stream in borked:
             try:
                 subs.remove(stream)
-            except ValueError:
+            except KeyError:
                 log.warning(
                     f'{stream._ctx.chan.uid} sub already removed!?'
                 )
@@ -429,7 +431,7 @@ async def maybe_open_samplerd(
     loglevel: str | None = None,
     **kwargs,
 
-) -> tractor._portal.Portal:  # noqa
+) -> tractor.Portal:  # noqa
     '''
     Client-side helper to maybe startup the ``samplerd`` service
     under the ``pikerd`` tree.
@@ -619,6 +621,14 @@ async def sample_and_broadcast(
             fqsn = f'{broker_symbol}.{brokername}'
             lags: int = 0
 
+            # TODO: speed up this loop in an AOT compiled lang (like
+            # rust or nim or zig) and/or instead of doing a fan out to
+            # TCP sockets here, we add a shm-style tick queue which
+            # readers can pull from instead of placing the burden of
+            # broadcast on solely on this `brokerd` actor. see issues:
+            # - https://github.com/pikers/piker/issues/98
+            # - https://github.com/pikers/piker/issues/107
+
             for (stream, tick_throttle) in subs.copy():
                 try:
                     with trio.move_on_after(0.2) as cs:
@@ -748,9 +758,6 @@ def frame_ticks(
             ticks_by_type[ttype].append(tick)
 
 
-# TODO: a less naive throttler, here's some snippets:
-# token bucket by njs:
-# https://gist.github.com/njsmith/7ea44ec07e901cb78ebe1dd8dd846cb9
 async def uniform_rate_send(
 
     rate: float,
@@ -760,8 +767,22 @@ async def uniform_rate_send(
     task_status: TaskStatus = trio.TASK_STATUS_IGNORED,
 
 ) -> None:
+    '''
+    Throttle a real-time (presumably tick event) stream to a uniform
+    transmissiom rate, normally for the purposes of throttling a data
+    flow being consumed by a graphics rendering actor which itself is limited
+    by a fixed maximum display rate.
 
-    # try not to error-out on overruns of the subscribed (chart) client
+    Though this function isn't documented (nor was intentially written
+    to be) a token-bucket style algo, it effectively operates as one (we
+    think?).
+
+    TODO: a less naive throttler, here's some snippets:
+    token bucket by njs:
+    https://gist.github.com/njsmith/7ea44ec07e901cb78ebe1dd8dd846cb9
+
+    '''
+    # try not to error-out on overruns of the subscribed client
     stream._ctx._backpressure = True
 
     # TODO: compute the approx overhead latency per cycle