build-aux/dist-vendor.sh: Add proper error checking

• build-aux/dist-vendor.sh: Add proper error checking:

  For automated release building the "dist" step should fail when any command in it fails. Since we can't rely on bash features, we need to adjust the script to fail on error. Make it clear that this is not a bash script and rewrite it to fail when any command fails.

• build-aux/dist-vendor.sh: Fix path quoting

  The path was not using quotes, meaning that this fails when any of the directories this is in has whitespace in its name. Fix all occurences of wrong quoting as called out by "shellcheck".