Fix git-lfs check to display error message

[kalakris]

Not sure if you accept patches on releases, but I don't think this applies to the latest version.

Summary

• Fixes the git-lfs installation check that silently exits without showing an error message
• The previous code used git lfs &>/dev/null followed by checking $?, but since the script uses set -e, the script exits immediately on the non-zero return code before reaching the print_error call

Root Cause

set -e  # line 11

git lfs &>/dev/null        # exits here with code 1
if [[ $? -ne 0 ]] ; then   # never reached
    print_error "..."      # never printed
fi

Fix

if ! git lfs &>/dev/null; then
    print_error "..."
fi

Fixes #151, #167, #176