Add various missing error handling

... as found by (golangci-lint run).

Note: this does not add (golangci-lint run) to the Makefile
to ensure the coding standard.

(BTW golangci-lint currently fails on structcheck, which doesn't
handle embedded structs, and that's a years-long known unfixed
limitation.)

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

integration/copy_test.go

@@ -86,10 +86,10 @@ func (s *CopySuite) TearDownSuite(c *check.C) {
 		os.RemoveAll(s.gpgHome)
 	}
 	if s.registry != nil {
-		s.registry.Close()
+		s.registry.tearDown(c)
 	}
 	if s.s1Registry != nil {
-		s.s1Registry.Close()
+		s.s1Registry.tearDown(c)
 	}
 	if s.cluster != nil {
 		s.cluster.tearDown(c)