buildman: Improve the toolchain progress/error output

Use colour to make it easier to see what is going on. Also print a message
before downloading a new toolchain. Mention --fetch-arch in the message that
is shown when there are no available toolchains, since this is the quickest
way to resolve the problem.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>