update: Optimize the error message

Signed-off-by: wujunwei <wjw3323@live.com>
(cherry picked from commit 4fcec24)

pkg/chart/dependency.go

@@ -54,7 +54,7 @@ type Dependency struct {
 // loaded.
 func (d *Dependency) Validate() error {
 	if d == nil {
-		return ValidationError("the dependency's item can not be nil")
+		return ValidationError("dependency cannot be an empty list")
 	}
 	d.Name = sanitizeString(d.Name)
 	d.Version = sanitizeString(d.Version)

pkg/chart/metadata.go

@@ -35,7 +35,7 @@ type Maintainer struct {
 // Validate checks valid data and sanitizes string characters.
 func (m *Maintainer) Validate() error {
 	if m == nil {
-		return ValidationError("the maintainer's item can not be nil")
+		return ValidationError("maintainer cannot be an empty list")
 	}
 	m.Name = sanitizeString(m.Name)
 	m.Email = sanitizeString(m.Email)

pkg/chart/metadata_test.go

@@ -82,7 +82,7 @@ func TestValidate(t *testing.T) {
 					nil,
 				},
 			},
-			ValidationError("the dependency's item can not be nil"),
+			ValidationError("dependency cannot be an empty list"),
 		},
 		{
 			&Metadata{
@@ -94,7 +94,7 @@ func TestValidate(t *testing.T) {
 					nil,
 				},
 			},
-			ValidationError("the maintainer's item can not be nil"),
+			ValidationError("maintainer cannot be an empty list"),
 		},
 		{
 			&Metadata{APIVersion: "v2", Name: "test", Version: "1.2.3.4"},