buildman: Write output even on fatal error

At present buildman does not write any output (to the 'out' and 'err)
files if the build terminates with a fatal error. This is to avoid adding
lots of spam to the logs.

However there are times when this is actually useful, such as when the
build fails for an obscure reason such as a Kconfig loop.

Update the logic to always write the output, so that the user gets a clue
as to what is happening.

Signed-off-by: Simon Glass <sjg@chromium.org>