x-error-msg HTTP header sometimes contains newlines which make it invalid

When the Subiquity server raises an exception in a HTTP request handler context, more often than not, the exception is sent back to the client in the body.

Additionally, the message of the exception (if any), is also copied as is in a x-error-msg HTTP header.

That said, HTTP headers must obey strict rules. While "\n" indicates the end of the current HTTP header, the aiohttp library rejects any header that has a "\r" or "\n" in its value:

ValueError: Newline or carriage return character detected in HTTP status message or header. This is a potential security issue.

As an example, any curtin.util.ProcessExecutionError exception will contain "\n" characters when converted into a string.