update error message for time travel (#359)

boa/environment.py

@@ -362,7 +362,7 @@ def time_travel(
         block_delta: int = 12,
     ) -> None:
         if (seconds is None) == (blocks is None):
-            raise ValueError("One of seconds or blocks should be set")
+            raise ValueError("One (and only one) of seconds or blocks should be set")
         if seconds is not None:
             blocks = seconds // block_delta
         else: