patman: Allow running a tool and returning the full result

patman: Allow running a tool and returning the full result

Add a new function which returns the entire result from running a tool,
not just stdout. Update Run() to use this and to return stdout on error,
if stderr is empty, since some unfortunate tools write their error
output to stdout rather than stderr.

Move building of the PATH to a separate function.

Make the exception catching more specific, to catch just ValueError, since
broad exceptions are a pain to debug.

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