Add ErrorHandler type to changelog

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- `ErrorHandler` type was added. Allow users to specify error handling function (e.g. Report error to error reporting service such as Honeybadger, Bugsnag, etc)
+
 ## [0.5.0] - 2020-02-23
 
 ### Changed

processor_test.go

@@ -197,7 +197,7 @@ func TestProcessorRetry(t *testing.T) {
 		}
 
 		if n != tc.wantErrCount {
-			t.Errorf("error handler was called %d, want %d", n, tc.wantErrCount)
+			t.Errorf("error handler was called %d times, want %d", n, tc.wantErrCount)
 		}
 	}
 }