build script ignores errors

[Min RK]

I recently pulled the dev-py3 image, and noticed that it doesn't actually have dolfin installed. I tracked it down to dolfin failing to build due to unicode issues and no locale being specified. The Docker builds should have failed, but don't because the build_fenics bash function ignores all errors and exits with status 0 regardless of success.

[Jack Hale]

You're quite right! The build script is in a pretty shonky state and needs a lot of improving/perhaps a total re-write.

[Jack Hale]

fenics-build (renamed build_fenics) no longer swallows errors.