HTTP errors from command-runner.sh should be returned as-is, not be changed into 500s

Describe the bug
Currently, the HgService.cs code uses response.EnsureSuccessStatusCode() to check for 200-299 status, and throws an exception if the status isn't 2xx. But the command-runner.sh script is carefully producing specific HTTP error codes like 404 or 400, which are being swallowed by EnsureSuccessStatusCode() as the exception means they all get turned into 500. I want to pass the codes from command-runner.sh back to the caller when appropriate (which is most, but not all, of the time).

Expected behavior
When command-runner.sh returns a 400 or 404, the HTTP client should receive that 400 or 404, not a 500. In some cases the 500 is appropriate, to be decided on a case-by-case basis.