The NewPrinter function is mostly borrowed from buildkit. However, at
some point, it seems that the implementations drifted.
This patch updates buildx to be more similar in behavior to it's
buildkit counterpart, specifically, it will explicitly fail if a TTY
output is requested using "--progress=tty", but the output is not
available.
To gracefully fallback to plain progress in this scenario,
"--progress=plain" is required.
Signed-off-by: Justin Chadwell <me@jedevc.com>