imgpkg with flag --json swallows error

**What steps did you take:**
Run `$ imgpkg push -b index.docker.io/username/my-bundle:v1.0.0 -f config/` with wrong username and got error:
```
dir: .
dir: .imgpkg
file: .imgpkg/images.yml
file: config.yml

imgpkg: Error: Writing 'index.docker.io/username/my-bundle:v1.0.0':
Writing image:
HEAD https://index.docker.io/v2/username/my-bundle/blobs/sha256:b622a27ef5b9d02070c45ddbf7a94670f6b41aea673ad91ce7c7315108e9585f: unexpected status code 401 Unauthorized (HEAD responses have no body, use GET for details)
```
Executed same command with flag `--json` and this time it exited without printing any error message or any other log.

**What happened:**
When we run imgpkg command with flag `--json` it did not print error message in case of failure.

**What did you expect:**
When we run imgpkg command with flag `--json` it should print error message in case of failure.

**Anything else you would like to add:**
[Additional information that will assist in solving the issue.]

**Environment:**

- imgpkg version (use `imgpkg --version`):
- Docker registry used (e.g. `Docker HUB`):
- OS (e.g. from `/etc/os-release`):

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