No helpful error output when private key provided requires password

[ghost]

To reproduce this issue, set the pipeline below and provide an SSH key that has a password in the private_key field. It's pretty quick to set up a new lock repo and add the generated public key as a deploy key on the repo (make sure to grant it write access). Then run the acquire-lock job. It errors out with absolutely no output. Obviously this wouldn't work since there's no way to provide a password -- I just discovered it when I pasted in the wrong key by mistake -- but we can hopefully save someone else the trouble by grepping their key for ENCRYPTED or something like that and printing an appropriate warning.

---
jobs:
- name: acquire-lock
  public: true
  plan:
  - put: test-pool
    params: {acquire: true}

- name: release-lock
  public: true
  plan:
  - put: test-pool
    params: {acquire: test-pool}

resources:
- name: test-pool
  type: pool
  source:
    uri: git@github.com:<REDACTED>/test-pool-resource.git
    branch: master
    pool: test-pool
    private_key: <private key that requires a password>

[concourse-bot]

Hi there!

We use Pivotal Tracker to provide visibility into what our team is working on. A story for this issue has been automatically created.

The current status is as follows:

• [ ] #121434747 No helpful error output when private key provided requires password

This comment, as well as the labels on the issue, will be automatically updated as the status in Tracker changes.

[jtarchie]

We should provide equivalent error messaging as the git resource.