all: Move away from pkg/errors, easy cases

github.com/pkg/errors is no longer getting updates, because Go 1.13
went with the more flexible errors.{As,Is} function. Use those instead:
errors from pkg/errors already support the Unwrap interface used by 1.13
error handling. Also:

* check for io.EOF with a straight ==. That value should not be wrapped,
  and the chunker (whose error is checked in the cases changed) does not
  wrap it.
* Give custom Error methods pointer receivers, so there's no ambiguity
  when type-switching since the value type will no longer implement error.
* Make restic.ErrAlreadyLocked private, and rename it to
  alreadyLockedError to match the stdlib convention that error type
  names end in Error.
* Same with rest.ErrIsNotExist => rest.notExistError.
* Make s3.Backend.IsAccessDenied a private function.

internal/backend/test/tests.go

@@ -361,8 +361,8 @@ func (s *Suite) TestListCancel(t *testing.T) {
 			return nil
 		})
 
-		if errors.Cause(err) != context.Canceled {
-			t.Fatalf("expected error not found, want %v, got %v", context.Canceled, errors.Cause(err))
+		if !errors.Is(err, context.Canceled) {
+			t.Fatalf("expected error not found, want %v, got %v", context.Canceled, err)
 		}
 	})
 
@@ -380,7 +380,7 @@ func (s *Suite) TestListCancel(t *testing.T) {
 			return nil
 		})
 
-		if errors.Cause(err) != context.Canceled {
+		if !errors.Is(err, context.Canceled) {
 			t.Fatalf("expected error not found, want %v, got %v", context.Canceled, err)
 		}
 
@@ -403,7 +403,7 @@ func (s *Suite) TestListCancel(t *testing.T) {
 			return nil
 		})
 
-		if errors.Cause(err) != context.Canceled {
+		if !errors.Is(err, context.Canceled) {
 			t.Fatalf("expected error not found, want %v, got %v", context.Canceled, err)
 		}
 
@@ -429,7 +429,7 @@ func (s *Suite) TestListCancel(t *testing.T) {
 			return nil
 		})
 
-		if errors.Cause(err) != context.DeadlineExceeded {
+		if !errors.Is(err, context.DeadlineExceeded) {
 			t.Fatalf("expected error not found, want %#v, got %#v", context.DeadlineExceeded, err)
 		}