buildman: don't fail --list-toolchains when toolchains fail

buildman: don't fail --list-toolchains when toolchains fail

When a toolchain invocation fails, an exception is thrown but not caught
which then aborts the entire toolchain detection process. To solve this,
request that exceptions not be thrown, since the toolchain init code
already error-checks the command result. This solves e.g.:

         - found '/usr/bin/winegcc'
Traceback (most recent call last):
...
Exception: Error running '/usr/bin/winegcc --version'

Change-Id: I579c72ab3b021e38b14132893c3375ea257c74f0
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
(formatted to 80cols)