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/sftp/sftp_test.go

@@ -20,7 +20,7 @@ func findSFTPServerBinary() string {
 	for _, dir := range strings.Split(rtest.TestSFTPPath, ":") {
 		testpath := filepath.Join(dir, "sftp-server")
 		_, err := os.Stat(testpath)
-		if !os.IsNotExist(errors.Cause(err)) {
+		if !errors.Is(err, os.ErrNotExist) {
 			return testpath
 		}
 	}