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.

cmd/restic/cmd_backup.go

@@ -144,7 +144,7 @@ func init() {
 func filterExisting(items []string) (result []string, err error) {
 	for _, item := range items {
 		_, err := fs.Lstat(item)
-		if err != nil && os.IsNotExist(errors.Cause(err)) {
+		if errors.Is(err, os.ErrNotExist) {
 			Warnf("%v does not exist, skipping\n", item)
 			continue
 		}

cmd/restic/cmd_mount.go

@@ -116,7 +116,7 @@ func runMount(opts MountOptions, gopts GlobalOptions, args []string) error {
 
 	mountpoint := args[0]
 
-	if _, err := resticfs.Stat(mountpoint); os.IsNotExist(errors.Cause(err)) {
+	if _, err := resticfs.Stat(mountpoint); errors.Is(err, os.ErrNotExist) {
 		Verbosef("Mountpoint %s doesn't exist\n", mountpoint)
 		return err
 	}

cmd/restic/global.go

@@ -297,7 +297,7 @@ func resolvePassword(opts GlobalOptions, envStr string) (string, error) {
 	}
 	if opts.PasswordFile != "" {
 		s, err := textfile.Read(opts.PasswordFile)
-		if os.IsNotExist(errors.Cause(err)) {
+		if errors.Is(err, os.ErrNotExist) {
 			return "", errors.Fatalf("%s does not exist", opts.PasswordFile)
 		}
 		return strings.TrimSpace(string(s)), errors.Wrap(err, "Readfile")
@@ -398,7 +398,7 @@ func ReadRepo(opts GlobalOptions) (string, error) {
 		}
 
 		s, err := textfile.Read(opts.RepositoryFile)
-		if os.IsNotExist(errors.Cause(err)) {
+		if errors.Is(err, os.ErrNotExist) {
 			return "", errors.Fatalf("%s does not exist", opts.RepositoryFile)
 		}
 		if err != nil {

cmd/restic/main.go

@@ -98,7 +98,7 @@ func main() {
 	err := cmdRoot.Execute()
 
 	switch {
-	case restic.IsAlreadyLocked(errors.Cause(err)):
+	case restic.IsAlreadyLocked(err):
 		fmt.Fprintf(os.Stderr, "%v\nthe `unlock` command can be used to remove stale locks\n", err)
 	case err == ErrInvalidSourceData:
 		fmt.Fprintf(os.Stderr, "Warning: %v\n", err)