posix: Return diskNotFound error rather than errVolumeNotFound (#1568)

Fixes #1559

posix.go

@@ -185,7 +185,8 @@ func (s fsStorage) getVolumeDir(volume string) (string, error) {
 		var volsInfo []VolInfo
 		volsInfo, err = getAllUniqueVols(s.diskPath)
 		if err != nil {
-			return volumeDir, errVolumeNotFound
+			// For any errors, treat it as disk not found.
+			return volumeDir, errDiskNotFound
 		}
 		for _, vol := range volsInfo {
 			// Verify if lowercase version of the volume is equal to

storage-errors.go

@@ -21,6 +21,9 @@ import "errors"
 // errDiskFull - cannot create volume or files when disk is full.
 var errDiskFull = errors.New("disk path full")
 
+// errDiskNotFount - cannot find the underlying configured disk anymore.
+var errDiskNotFound = errors.New("disk not found")
+
 // errFileNotFound - cannot find the file.
 var errFileNotFound = errors.New("file not found")