Add `MarketNotFound` exc and improve binance fqme error

Add a `MarketNotFound(SymbolNotFound)` subclass for
mkt-pair-specific lookup failures; use it in binance
`get_mkt_info()` with a detailed expected-form hint.

Deats,
- add `MarketNotFound` in `brokers/_util.py`.
- re-export from `brokers/__init__.py`.
- binance `feed.py`: swap `SymbolNotFound` import
  for `MarketNotFound`; build `expected` string
  showing the `<pair>.<venue>.<broker>` format
  and suggest `".spot."` if venue is missing.

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

piker/brokers/__init__.py

@@ -31,6 +31,7 @@ from piker.log import (
 from ._util import (
     BrokerError,
     SymbolNotFound,
+    MarketNotFound as MarketNotFound,
     NoData,
     DataUnavailable,
     DataThrottle,

piker/brokers/_util.py

@@ -66,6 +66,10 @@ class SymbolNotFound(BrokerError):
     "Symbol not found by broker search"
 
 
+class MarketNotFound(SymbolNotFound):
+    "Mkt-pair not found by broker search"
+
+
 # TODO: these should probably be moved to `.tsp/.data`?
 class NoData(BrokerError):
     '''

piker/brokers/binance/feed.py

@@ -48,7 +48,7 @@ import tractor
 from piker.brokers import (
     open_cached_client,
     NoData,
-    SymbolNotFound,
+    MarketNotFound,
 )
 from piker._cacheables import (
     async_lifo_cache,
@@ -325,9 +325,21 @@ async def get_mkt_info(
     venue_lower: str = venue.lower()
 
     if not venue:
-        raise SymbolNotFound(
+        if expiry:
+            expiry = f'.{expiry}'
+        expected: str = (
+            f'{mkt_ep}'
+            f'.<venue>'
+            f'{expiry}'
+            f'.{broker}'
+        )
+        raise MarketNotFound(
             f'Invalid or missing .<venue> part in fqme?\n'
+            f'\n'
             f'fqme: {fqme!r}\n'
+            f'expected-form>> {expected}\n'
+            f'\n'
+            f'Maybe you are missing a ".spot." ?\n'
         )
 
     # XXX TODO: we should change the usdtm_futes name to just