[errors] Greatly simplify common/errors package.

common/tsmon/config.go

Index: common/tsmon/config.go
diff --git a/common/tsmon/config.go b/common/tsmon/config.go
index 26d63c08725cc54ff2fda2a02fdb1df17a592ef8..1fc6e2e0e48d5e996415afb7e104627e6213486c 100644
--- a/common/tsmon/config.go
+++ b/common/tsmon/config.go
@@ -37,7 +37,7 @@ func loadConfig(path string) (config, error) {
 
 		decoder := json.NewDecoder(file)
 		if err = decoder.Decode(&ret); err != nil {
-			return ret, errors.Annotate(err).Reason("failed to decode file").Err()
+			return ret, errors.Annotate(err, "failed to decode file").Err()
 		}
 		return ret, nil
 
@@ -48,7 +48,7 @@ func loadConfig(path string) (config, error) {
 
 	default:
 		// An unexpected failure occurred.
-		return ret, errors.Annotate(err).Reason("failed to open file").Err()
+		return ret, errors.Annotate(err, "failed to open file").Err()
 	}
 }