add exception error prelude.

diff --git a/src/prolog/io.rs b/src/prolog/io.rs
index f971e4db0deb682399b7189b4c1cbc487112c729..5af2a61b91aec2060d1819095d9eae129944ac29 100644 (file)
--- a/src/prolog/io.rs
+++ b/src/prolog/io.rs
@@ -404,7 +404,7 @@ Each predicate must have the same name and arity.";
 }
 
 fn error_string(e: &String) -> String {
-    format!("error: {}", e)
+    format!("error: exception thrown: {}", e)
 }
 
 pub fn print(wam: &mut Machine, result: EvalSession) {