Add wrap verb to formatted error outputs

[mkatychev]

• Replaces %v formatting of errors using fmt.Errorf with %w.
• In cases where an Unwrap method is not used, %w behaves in the exact same way as %v.

Reference:

In Go 1.13, the fmt.Errorf function supports a new %w verb. When this verb is present, the error returned by fmt.Errorf will have an Unwrap method returning the argument of %w, which must be an error. In all other ways, %w is identical to %v.

This would allow returned errors such as this one to check and handle cases when the underlying error is os.ErrNotExist using os.IsNotExist(err):
https://github.com/mozilla/sops/blob/8e21de8dbc750f050a54d63209f2f1f93477168c/decrypt/decrypt.go#L21-L24

[autrilla]

Thanks!