add more tests for fmt.Errorf and improve test function error provide…

…r names

tests/fault_test.go

@@ -117,3 +117,25 @@ func TestFlattenStdlibErrorfWrappedError(t *testing.T) {
 	a.Equal("", e2.Message)
 	a.Contains(e2.Location, "test_callers.go:11")
 }
+
+func TestFlattenStdlibErrorfWrappedExternalError(t *testing.T) {
+	a := assert.New(t)
+	err := errorCaller(6)
+	chain := fault.Flatten(err)
+
+	a.ErrorContains(err, "external error wrapped with errorf: stdlib external error")
+	a.ErrorContains(chain.Root, "stdlib external error")
+	a.Len(chain.Errors, 3)
+
+	e0 := chain.Errors[0]
+	a.Equal("errorf wrapped external: external error wrapped with errorf: stdlib external error", e0.Message)
+	a.Contains(e0.Location, "test_callers.go:29")
+
+	e1 := chain.Errors[1]
+	a.Equal("failed to call function", e1.Message)
+	a.Contains(e1.Location, "test_callers.go:20")
+
+	e2 := chain.Errors[2]
+	a.Equal("", e2.Message)
+	a.Contains(e2.Location, "test_callers.go:11")
+}

tests/root.go

@@ -28,6 +28,15 @@ func rootCause(kind int) error {
 		return fault.New("fault root cause error")
 	} else if kind == 5 {
 		return fmt.Errorf("errorf wrapped: %w", errSentinelStdlib)
+	} else if kind == 6 {
+		err := externalError()
+		return fmt.Errorf("errorf wrapped external: %w", err)
 	}
 	return nil
 }
+
+var errExternalStdlib = errors.New("stdlib external error")
+
+func externalError() error {
+	return fmt.Errorf("external error wrapped with errorf: %w", errExternalStdlib)
+}

tests/test_callers.go

@@ -6,24 +6,24 @@ import (
 )
 
 func errorCaller(kind int) error {
-	err := errorCallerMid(kind)
+	err := errorCallerFromMiddleOfChain(kind)
 	if err != nil {
 		return fault.Wrap(err)
 	}
 
 	return nil
 }
 
-func errorCallerMid(kind int) error {
-	err := errorCallerDeep(kind)
+func errorCallerFromMiddleOfChain(kind int) error {
+	err := errorProducerFromRootCause(kind)
 	if err != nil {
 		return fault.Wrap(err, fmsg.With("failed to call function"))
 	}
 
 	return nil
 }
 
-func errorCallerDeep(kind int) error {
+func errorProducerFromRootCause(kind int) error {
 	err := rootCause(kind)
 	if err != nil {
 		return fault.Wrap(err)