[#277] getsvc: Rename and reorder code

Rename execCtx to request.
Move code to appropriate files.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/services/object/get/errors.go

@@ -0,0 +1,10 @@
+package getsvc
+
+import "errors"
+
+var (
+	errRangeZeroLength      = errors.New("zero range length")
+	errRangeOverflow        = errors.New("range overflow")
+	errChildWithEmptyParent = errors.New("received child with empty parent")
+	errParentAddressDiffers = errors.New("parent address in child object differs")
+)