devtool: properly handle bb.build.FuncFailed when extracting source

devtool: properly handle bb.build.FuncFailed when extracting source

When we run the tasks required to extract the source for a recipe (e.g.
within "devtool modify" or "devtool extract") if one of those tasks
fails you get a bb.build.FuncFailed exception; handle this properly so
you don't see a traceback.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>