### Summary

`ansible-galaxy collection install` makes a lot of requests to galaxy.ansible.com and can occasionally hit the rate limit. When that happens, galaxy.ansible.com will return a 520 http code and `ansible-galaxy` fails to download the collection and exits. Ideally when the client encounters a rate limiting http code (either 520, or 429), it should wait, slow down the request rate and try again rather than exiting.

More information is available here: https://github.com/ansible/galaxy/issues/2429 

### Issue Type

Bug Report

### Component Name

ansible-galaxy

### Ansible Version

```console
$ ansible --version
root@ubuntu-s-1vcpu-1gb-nyc3-01:~# ansible --version
ansible 2.10.7
  config file = None
  configured module search path = ['/root/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/local/lib/python3.8/dist-packages/ansible
  executable location = /usr/local/bin/ansible
  python version = 3.8.5 (default, Jan 27 2021, 15:41:15) [GCC 9.3.0]
```


### Configuration

```console
$ ansible-config dump --only-changed
root@ubuntu-s-1vcpu-1gb-nyc3-01:~# ansible-config dump --only-changed
root@ubuntu-s-1vcpu-1gb-nyc3-01:~#
```


### OS / Environment

All

### Steps to Reproduce

Run `ansible-galaxy collection install `amazon.aws`. If your internet is fast enough, this will occasionally fail when galaxy.ansible.com returns a 520 error code.

### Expected Results

Collection should be installed.

### Actual Results

```console
`ansible-galaxy` encounters a 429 or 520 http code and exits.
```


### Code of Conduct

I agree to follow the Ansible Code of Conduct


'finished_at': 'some_time',
            'state': 'failed',
            'error': {
                'code': 'GW001',
                'description': u'Becäuse I said so!',
            },
            'messages': [
                {
                    'level': 'error',
                    'message': u'Somé error',
                },
                {
                    'level': 'warning',
                    'message': u'Some wärning',
                },
                {
                    'level': 'info',
                    'message': u'Somé info',
                },
            ],
        }))),
    ]
    monkeypatch.setattr(galaxy_api, 'open_url', mock_open)
    mock_display = MagicMock()
    monkeypatch.setattr(Display, 'display', mock_display)
    mock_vvv = MagicMock()
    monkeypatch.setattr(Display, 'vvv', mock_vvv)
    mock_warn = MagicMock()
    monkeypatch.setattr(Display, 'warning', mock_warn)
    mock_err = MagicMock()