Fix error when using upstream for HTTP request

[ImpostorKeanu]

1. What does this change do, exactly?

This change properly scopes an error value.

Without scoping the error, any request with a Host header value that doesn't have a port will result in a bad gateway response when the handler is configured with an upstream.

This is because the error is shadowed at this point during execution.

Here's an example route configuration that would error out:

- handler: forward_proxy
  upstream: http://my.proxy.com

Here's a command that would induce an error:

# assumes server listens on localhost 20000
curl -vv -x http://127.0.0.1:20000 http://ifconfig.io

2. Please link to the relevant issues.

N/A

3. Which documentation changes (if any) need to be made because of this PR?

N/A

4. Checklist

• I have written tests and verified that they fail without my change
• I made pull request as minimal and simple as possible. If change is not small or additional dependencies are required, I opened an issue to propose and discuss the design first
• I have squashed any insignificant commits
• This change has comments for package types, values, functions, and non-obvious lines of code