rados: modify validate() to return ErrInvalidIOContext error

If ioctx is not ready to be used then validate() should
return ErrInvalidIOContext error.

Signed-off-by: Mudit Agarwal <muagarwa@redhat.com>

rados/errors.go

@@ -54,6 +54,9 @@ var (
 	// ErrEmptyArgument may be returned if a function argument is passed
 	// a zero-length slice or map.
 	ErrEmptyArgument = errors.New("Argument must contain at least one item")
+	// ErrInvalidIOContext may be returned if an api call requires an IOContext
+	// but IOContext is not ready for use.
+	ErrInvalidIOContext = errors.New("IOContext is not ready for use")
 )
 
 // Public radosErrors:

rados/ioctx.go

@@ -96,7 +96,7 @@ type IOContext struct {
 // with ceph C calls.
 func (ioctx *IOContext) validate() error {
 	if ioctx.ioctx == nil {
-		return ErrNotConnected
+		return ErrInvalidIOContext
 	}
 	return nil
 }