'pass' based credential helper returns success when cred doesn't exist

[nicks]

Repro steps:

echo "garbage" | go run ./pass/cmd/main.go get

Expected:
Exit code 1

Actual:
Exit code 0
{"ServerURL":"garbage","Username":"","Secret":""}

This is the root of cryptic errors like:

WARNING: could not erase credentials: error erasing credentials - err: exit status 1, out: `exit status 1: Error: docker-credential-helpers/[CRED_FOLDER_NAME] is not in the password store.` 

[ericpromislow]

The thing I find strange is that calling the underlying function pass show security would write Error: security is not in the password store to stderr and exit with status 1. But if I remember looking at this code a while ago, it's walking the raw filesystem.

This is related to the other problem that you can't use docker-credential-pass list to determine if the docker password-store has even been initialized because it returns {}, even if pass (1) hasn't been installed.