improve error message on plugin install

Signed-off-by: Philipp Stehle <philipp.stehle@sap.com>

pkg/plugin/installer/local_installer.go

@@ -22,6 +22,9 @@ import (
 	"github.com/pkg/errors"
 )
 
+// ErrPluginNotAFolder indicates that the plugin path is not a folder.
+var ErrPluginNotAFolder = errors.New("expected plugin to be a folder")
+
 // LocalInstaller installs plugins from the filesystem.
 type LocalInstaller struct {
 	base
@@ -43,6 +46,14 @@ func NewLocalInstaller(source string) (*LocalInstaller, error) {
 //
 // Implements Installer.
 func (i *LocalInstaller) Install() error {
+	stat, err := os.Stat(i.Source)
+	if err != nil {
+		return err
+	}
+	if !stat.IsDir() {
+		return ErrPluginNotAFolder
+	}
+
 	if !isPlugin(i.Source) {
 		return ErrMissingMetadata
 	}

pkg/plugin/installer/local_installer_test.go

@@ -48,3 +48,19 @@ func TestLocalInstaller(t *testing.T) {
 	}
 	defer os.RemoveAll(filepath.Dir(helmpath.DataPath())) // helmpath.DataPath is like /tmp/helm013130971/helm
 }
+
+func TestLocalInstallerNotAFolder(t *testing.T) {
+	source := "../testdata/plugdir/good/echo/plugin.yaml"
+	i, err := NewForSource(source, "")
+	if err != nil {
+		t.Fatalf("unexpected error: %s", err)
+	}
+
+	err = Install(i)
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if err != ErrPluginNotAFolder {
+		t.Fatalf("expected error to equal: %q", err)
+	}
+}