[composition] linktime_composition should have a more obvious error message on Windows

[ivanpauno]

Bug report

Required Info:

• Operating System:
  • Windows 10
• Installation type:
  • binary/source
• Version or commit hash:
  • 113e6ea
• DDS implementation:
  • cyclonedds (shouldn't matter)
• Client library (if applicable):
  • rclcpp

Steps to reproduce issue

# source your ros2 workspace
ros2 run composition linktime_composition

Expected behavior

See output similar to linux:

[INFO] [1619635824.514092711] [linktime_composition]: Load library 
[INFO] [1619635824.514176377] [linktime_composition]: Instantiate class rclcpp_components::NodeFactoryTemplate<composition::Client>
[INFO] [1619635824.524012232] [linktime_composition]: Instantiate class rclcpp_components::NodeFactoryTemplate<composition::Listener>
[INFO] [1619635824.528914045] [linktime_composition]: Instantiate class rclcpp_components::NodeFactoryTemplate<composition::Server>
[INFO] [1619635824.533364960] [linktime_composition]: Instantiate class rclcpp_components::NodeFactoryTemplate<composition::Talker>
[INFO] [1619635825.538035457] [talker]: Publishing: 'Hello World: 1'
[INFO] [1619635825.539072214] [listener]: I heard: [Hello World: 1]
[INFO] [1619635826.526498042] [Server]: Incoming request: [a: 2, b: 3]
[INFO] [1619635826.527478895] [Client]: Got result: [5]
[INFO] [1619635826.537941787] [talker]: Publishing: 'Hello World: 2'
[INFO] [1619635826.538641392] [listener]: I heard: [Hello World: 2]
[INFO] [1619635827.538077435] [talker]: Publishing: 'Hello World: 3'
[INFO] [1619635827.538899874] [listener]: I heard: [Hello World: 3]
[INFO] [1619635828.525274899] [Server]: Incoming request: [a: 2, b:
....

Actual behavior

No output.

Additional information

Linktime composition doesn't work on Windows, as commented in the test

demos/composition/test/test_linktime_composition.py.in

Lines 47 to 48 in d512b5c

	if os.name == 'nt':
	print('The link time registration of classes does not work on Windows')

We should either not generate that executable on Windows, or make it print an obvious error message to the user.