Fail with more descriptive error when processing message involving inactive coin.

basicswap/util/__init__.py

@@ -25,6 +25,13 @@ class AutomationConstraint(ValueError):
     pass
 
 
+class InactiveCoin(Exception):
+    def __init__(self, coinid):
+        self.coinid = coinid
+    def __str__(self):
+        return str(self.coinid)
+
+
 def ensure(v, err_string):
     if not v:
         raise ValueError(err_string)