Always direct error and diagnostic output to the stderr (buildtools, 4654413)

Without this it may be harder to diagnose script failures (for example, in the ReleaseNotes.sh script we normally redirect the output to a file; in that case any error messages wind up in the file rather than in the Xcode build window where we want to see them).

Signed-off-by: Greg Hurrell <greg@hurrell.net>