hugofs: Fix golint issues

Fix godoc issues and the following:

hugofs/noop_fs.go:25:2: error var noOpErr should have name of the form errFoo

hugofs/noop_fs.go

@@ -22,24 +22,27 @@ import (
 )
 
 var (
-	noOpErr          = errors.New("this is a filesystem that does nothing and this operation is not supported")
+	errNoOp          = errors.New("this is a filesystem that does nothing and this operation is not supported")
 	_       afero.Fs = (*noOpFs)(nil)
-	NoOpFs           = &noOpFs{}
+
+	// NoOpFs provides a no-op filesystem that implements the afero.Fs
+	// interface.
+	NoOpFs = &noOpFs{}
 )
 
 type noOpFs struct {
 }
 
 func (fs noOpFs) Create(name string) (afero.File, error) {
-	return nil, noOpErr
+	return nil, errNoOp
 }
 
 func (fs noOpFs) Mkdir(name string, perm os.FileMode) error {
-	return noOpErr
+	return errNoOp
 }
 
 func (fs noOpFs) MkdirAll(path string, perm os.FileMode) error {
-	return noOpErr
+	return errNoOp
 }
 
 func (fs noOpFs) Open(name string) (afero.File, error) {
@@ -51,15 +54,15 @@ func (fs noOpFs) OpenFile(name string, flag int, perm os.FileMode) (afero.File,
 }
 
 func (fs noOpFs) Remove(name string) error {
-	return noOpErr
+	return errNoOp
 }
 
 func (fs noOpFs) RemoveAll(path string) error {
-	return noOpErr
+	return errNoOp
 }
 
 func (fs noOpFs) Rename(oldname string, newname string) error {
-	return noOpErr
+	return errNoOp
 }
 
 func (fs noOpFs) Stat(name string) (os.FileInfo, error) {
@@ -71,9 +74,9 @@ func (fs noOpFs) Name() string {
 }
 
 func (fs noOpFs) Chmod(name string, mode os.FileMode) error {
-	return noOpErr
+	return errNoOp
 }
 
 func (fs noOpFs) Chtimes(name string, atime time.Time, mtime time.Time) error {
-	return noOpErr
+	return errNoOp
 }