Arc prompts should use STDERR


Observed Behavior:

If you run a script containing a command like arc diff --create --message-file /tmp/foobar, and capture the results, the script will appear to hang if arcanist is prompting for a question like “You have not specified any reviewers. Continue anyway?”. This is because phutil_console_prompt() prints it output to STDOUT. If the function printed prompts to STDERR instead, they would always be visible to the user.