More graceful handling of bad match functions

config.go

@@ -51,12 +51,18 @@ func MakeMatchFunc(rule string) MatchFunc {
 			"hostname": dest.Hostname(),
 			"url":      dest,
 		}
-		matchFunc, err := expr.Eval(rule, env)
+		matchFuncExpr, err := expr.Eval(rule, env)
 		if err != nil {
-			panic(err)
+			fmt.Printf("Error evaluating rule %s: %v", rule, err)
+			return false
 		}
 
-		return matchFunc.(MatchFunc)(dest)
+		matchFunc, ok := matchFuncExpr.(MatchFunc)
+		if !ok {
+			fmt.Printf("Error evaluating rule %s. Did not evaluate to a MatchFunc.", rule)
+			return false
+		}
+		return matchFunc(dest)
 	}
 }