### 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


assert mock_call.mock_calls[0][2]['headers']['Content-type'].startswith(
        'multipart/form-data; boundary=')
    assert mock_call.mock_calls[0][2]['args'].startswith(b'--')
    assert mock_call.mock_calls[0][2]['method'] == 'POST'
    assert mock_call.mock_calls[0][2]['auth_required'] is True
@pytest.mark.parametrize('api_version, collection_url, response, expected', [
    ('v2', 'collections', {},
     'Error when publishing collection to test (%s) (HTTP Code: 500, Message: msg Code: Unknown)'),
    ('v2', 'collections', {
        'message': u'Galaxy error messäge',
        'code': 'GWE002',
    }, u'Error when publishing collection to test (%s) (HTTP Code: 500, Message: Galaxy error messäge Code: GWE002)'),
    ('v3', 'artifact/collections', {},
     'Error when publishing collection to test (%s) (HTTP Code: 500, Message: msg Code: Unknown)'),
    ('v3', 'artifact/collections', {
        'errors': [
            {
                'code': 'conflict.collection_exists',
                'detail': 'Collection "mynamespace-mycollection-4.1.1" already exists.',
                'title': 'Conflict.',
                'status': '400',
            },
            {
                'code': 'quantum_improbability',
                'title': u'Rändom(?) quantum improbability.',
                'source': {'parameter': 'the_arrow_of_time'},
                'meta': {'remediation': 'Try again before'},
            },
        ],
    }, u'Error when publishing collection to test (%s) (HTTP Code: 500, Message: Collection '