### 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


""" Error for bad Galaxy server responses. """
    def __init__(self, http_error, message):
        super(GalaxyError, self).__init__(message)
        self.http_code = http_error.code
        self.url = http_error.geturl()
        try:
            http_msg = to_text(http_error.read())
            err_info = json.loads(http_msg)
        except (AttributeError, ValueError):
            err_info = {}
        url_split = self.url.split('/')
        if 'v2' in url_split:
            galaxy_msg = err_info.get('message', http_error.reason)
            code = err_info.get('code', 'Unknown')
            full_error_msg = u"%s (HTTP Code: %d, Message: %s Code: %s)" % (message, self.http_code, galaxy_msg, code)
        elif 'v3' in url_split:
            errors = err_info.get('errors', [])
            if not errors:
                errors = [{}]  
            message_lines = []
            for error in errors:
                error_msg = error.get('detail') or error.get('title') or http_error.reason