Merge pull request #89 from alext/improve_succeed_output

Improve Succeed() failure message.
diff --git a/matchers/succeed_matcher.go b/matchers/succeed_matcher.go
index c162b67..f7dd853 100644
--- a/matchers/succeed_matcher.go
+++ b/matchers/succeed_matcher.go
@@ -22,7 +22,7 @@
 }
 
 func (matcher *SucceedMatcher) FailureMessage(actual interface{}) (message string) {
-	return fmt.Sprintf("Expected success, but got an error:\n%s", format.Object(actual, 1))
+	return fmt.Sprintf("Expected success, but got an error:\n%s\n%s", format.Object(actual, 1), format.IndentString(actual.(error).Error(), 1))
 }
 
 func (matcher *SucceedMatcher) NegatedFailureMessage(actual interface{}) (message string) {