Fix a 500 error in the SSH host keys lookup action

What does this MR do?

Catches an Addressable::URI exception raised when parsing the URL passed to the ssh_host_keys action, allowing the user to get a 400, rather than 500, response.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

500 errors are bad

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #3312 (closed)