DeleteConflict error with custom policy file

With the same call to `chalice deploy` I am deploying a REST API and a lambda function. This is so that I can run the Lambda function asynchronously after responding to the request.

```python
@app.route('/v1/lambdaTest')
def test_endpoint():
_ = lambda_client.invoke(
FunctionName='mvp-api-dev-testFunction',
InvocationType='Event',
Payload="{}"
)

return {'status_code': 200}

@app.lambda_function(name='testFunction')
def test_lambda_function(event, context):
time.sleep(20)
```
The deployed API Lambda does not have permission automatically to invoke `api-dev-testFunction`. This can be fixed by updating the policy in the AWS console but it must be done on every deployment.

To avoid this, I created a policy file `.chalice/policy-dev.json` with the policy configuration, which is like this:
```json
{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Action": [
"logs:CreateLogGroup",
"logs:CreateLogStream",
"logs:PutLogEvents"
],
"Resource": "arn:*:logs:*:*:*"
},
{
"Sid": "InvokePermission",
"Effect": "Allow",
"Action": [
"lambda:InvokeFunction"
],
"Resource": "*"
}
]
}
```
The `.chalice/config.json` looks like this, with `autogen_policy` set to false, and pointing to the above policy file `policy-dev.json`:
```json
{
"version": "2.0",
"app_name": "mvp-api",
"stages": {
"dev": {
"lambda_memory_size": 3008,
"api_gateway_stage": "api",
"autogen_policy": false,
"iam_policy_file": "policy-dev.json"
}
}
}
```

After `chalice deploy`, the API works as expected, but an error is reported in the console. Here is the stack trace:
```linux
(-----) user@system:~/projects/mvp-api$ chalice deploy
/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/_distutils_hack/__init__.py:30: UserWarning: Setuptools is replacing distutils.
warnings.warn("Setuptools is replacing distutils.")
Creating deployment package.
Reusing existing deployment package.
Updating policy for IAM role: mvp-api-dev-testFunction
Updating lambda function: mvp-api-dev-testFunction
Updating policy for IAM role: mvp-api-dev-api_handler
Updating lambda function: mvp-api-dev
Updating rest API
Deleting IAM role: mvp-api-dev
Traceback (most recent call last):
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/chalice/deploy/deployer.py", line 376, in deploy
return self._deploy(config, chalice_stage_name)
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/chalice/deploy/deployer.py", line 392, in _deploy
self._executor.execute(plan)
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/chalice/deploy/executor.py", line 42, in execute
getattr(self, '_do_%s' % instruction.__class__.__name__.lower(),
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/chalice/deploy/executor.py", line 55, in _do_apicall
result = method(**final_kwargs)
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/chalice/awsclient.py", line 1062, in delete_role
client.delete_role(RoleName=name)
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/botocore/client.py", line 508, in _api_call
return self._make_api_call(operation_name, kwargs)
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/botocore/client.py", line 915, in _make_api_call
raise error_class(parsed_response, operation_name)
botocore.errorfactory.DeleteConflictException: An error occurred (DeleteConflict) when calling the DeleteRole operation: Cannot delete entity, must detach all policies first.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/chalice/cli/__init__.py", line 636, in main
return cli(obj={})
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/click/core.py", line 1130, in __call__
return self.main(*args, **kwargs)
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/click/core.py", line 1055, in main
rv = self.invoke(ctx)
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/click/core.py", line 1657, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/click/core.py", line 1404, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/click/core.py", line 760, in invoke
return __callback(*args, **kwargs)
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/click/decorators.py", line 26, in new_func
return f(get_current_context(), *args, **kwargs)
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/chalice/cli/__init__.py", line 189, in deploy
deployed_values = d.deploy(config, chalice_stage_name=stage)
File "/home/-----/anaconda3/envs/mvp38/lib/python3.8/site-packages/chalice/deploy/deployer.py", line 378, in deploy
raise ChaliceDeploymentError(e)
chalice.deploy.deployer.ChaliceDeploymentError: ERROR - While deploying your chalice application, received the following error:

An error occurred (DeleteConflict) when calling the DeleteRole operation:
Cannot delete entity, must detach all policies first.
```
Any ideas on the meaning of this error, and how to correct it?

Many thanks to you!

Start with deployer.py and executor.py, then follow the awsclient.py delete_role call shown in the traceback. Reproduce the failure with the provided .chalice/config.json and policy-dev.json, and inspect the deployment sequence around policy updates and role deletion. Done means chalice deploy completes without DeleteConflict while retaining the required Lambda invocation permission.

Written by the indexing model from the issue text.