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/test/helpers.go

@@ -175,7 +175,7 @@ func ResetReadOnly(t testing.TB, dir string) {
 
 		return nil
 	})
-	if os.IsNotExist(errors.Cause(err)) {
+	if errors.Is(err, os.ErrNotExist) {
 		err = nil
 	}
 	OK(t, err)
@@ -186,7 +186,7 @@ func ResetReadOnly(t testing.TB, dir string) {
 func RemoveAll(t testing.TB, path string) {
 	ResetReadOnly(t, path)
 	err := os.RemoveAll(path)
-	if os.IsNotExist(errors.Cause(err)) {
+	if errors.Is(err, os.ErrNotExist) {
 		err = nil
 	}
 	OK(t, err)