Clean up error message for incorrect passphrase

[riyazdf]

We get weird (MISSING) delimiters and other hard-to-read output when failing to login with correct credentials against a notary-server with auth (like https://notary.docker.io)

notary 🐳 $ bin/notary -s https://<URL> publish <REPO>
Pushing changes to <REPO>
Enter username: <USERNAME>
Enter password:

* fatal: token auth attempt for registry: https://<URL>/auth/token?account=<USERNAME>&scope=repository%!A(MISSING)d<URL>%!F(MISSING)<ORG>%!F(MISSING)<REPO>%!A(MISSING)push%!C(MISSING)pull&service=<URL> request failed with status: 401 Unauthorized

I've added in <URL>, <REPO>, <USERNAME>, and <ORG> myself, but the %!F(MISSING) output is straight from the terminal

[riyazdf]

Closed with #972