add status code to err msg, fix #21

[sblack4]

• Please check if the PR includes the following

• [ ] Tests for the changes have been added (if applicable)

• [ ] Docs have been added / updated (if applicable)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

  21 I added the status code to the FilingUnavailableError options

• What is the current behavior? (You can also link to an open issue here) No status code :(

• What is the new behavior (if this is a feature change)? Message with status code :)

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?) No

• Other information: Thank you for this library. It's saved me some work

[esonderegger]

This seems like it could be helpful. Happy to hear you're using the iter_http method!

Thanks for suggesting this!