Add the error body to the URL config GET error output

### Use Case

When fetching a configuration file from a URL, include the error body in the error output to make it easier to identify why fetching a configuration failed.

### Expected behavior

When fetching a configuration from a URL and it fails, I expect to be able to see the error message related to the failure.

### Actual behavior

Currently, when fetching a configuration from a URL, only the response code is displayed in the Telegraf output, not the response body. The actual error message is typically provided in the response body.

### Additional info

_No response_

Start by locating the Telegraf entry point that fetches configuration files from URLs and the existing error-output handling. Reproduce a failed URL fetch, inspect how only the response code is reported, and add coverage showing that the response body is included in the resulting error message.

Written by the indexing model from the issue text.