add error handling to mine_vanity_key function

nostr/key.py

@@ -109,8 +109,10 @@ class PrivateKey:
         return self.raw_secret == other.raw_secret
 
 def mine_vanity_key(prefix: str = None, suffix: str = None) -> PrivateKey:
+    if prefix is None and suffix is None:
+        raise ValueError("Expected at least one of 'prefix' or 'suffix' arguments")
+
     while True:
-        attempts += 1
         sk = PrivateKey()
         if prefix is not None and not sk.public_key.bech32()[5:5+len(prefix)] == prefix:
             continue