Better log on guess_build_system error

Description

On a project have a bad pyproject.toml file without the line

build-backend = "poetry.core.masonry.api"

the idea is to clearly indicate this line is missing

Implementation ideas

      if [[ "$build_backend" ]]
      then
        case "$build_backend" in
        poetry.core.masonry.api)          
          log_info "--- Build system auto-detected: PEP 517 with Poetry backend"
          export PYTHON_BUILD_SYSTEM="poetry"
          return
          ;;
        setuptools.build_meta)          
          log_info "--- Build system auto-detected: PEP 517 with Setuptools backend"
          export PYTHON_BUILD_SYSTEM="setuptools"
          return
          ;;

add this

        "")
          log_error "--- Build system auto-detected: PEP 517 but file pyproject.toml don't have build-backend = \"XXX\""
          exit 1
          ;;

rest of the template code

        *)
          log_error "--- Build system auto-detected: PEP 517 with unsupported backend \\e[33;1m${build_backend}\\e[0m: please read template doc"
          exit 1
          ;;
        esac
      fi
    fi