Remove agnostic lib

[dirk-thomas]

Since the rclpy lib is now agnostic to the RMW impl. there is no need to keep some functions in a separate library. And since the loaded Python extension is always the same I removed all the fancy loading logic around it.

The second commit addresses a test failure. Instead of passing a known invalid parameter to the C function raise a ValueError instead.

fixes the failing rclpytestst (http://ci.ros2.org/view/nightly/job/nightly_win_deb/lastCompletedBuild/testReport/(root)/projectroot/rclpytests/).

[wjwwood]

Isn't this redundant with the ament_target_dependencies call below?

[dirk-thomas]

Absolutely, I moved it from the other library target and noticed the duplicate. Removed in a699313.

[wjwwood]

Shouldn't this have a message?

[dirk-thomas]

It was only used in one bizarre test (which modified the internal handle of a node instance) so I thought it isn't a real world case. Added a message in 117eac9.

[wjwwood]

Is this necessary?

[dirk-thomas]

Not necessary but recommended in PEP 8 and some linters will warn about it:

When catching exceptions, mention specific exceptions whenever possible instead of using a bare except:

[wjwwood]

Unfortunately I didn't catch this before, but the removal of the proxy object machinery has caused a regression in the check that rclpy.init() has been called before using other functions. Now you get a general RuntimeError rather than the more specific rclpy.exceptions.NotInitializedException when doing something like this:

In [1]: import rclpy

In [2]: node = rclpy.create_node('foo')
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-25-e3302d2d805e> in <module>()
----> 1 node = rclpy.create_node('foo')

/Users/william/ros2_ws/install/lib/python3.6/site-packages/rclpy/__init__.py in create_node(node_name, namespace)
     26
     27 def create_node(node_name, *, namespace=None):
---> 28     node_handle = _rclpy.rclpy_create_node(node_name, namespace or '')
     29     return Node(node_handle)
     30

RuntimeError: Failed to create node: rcl_init() has not been called, at /Users/william/ros2_ws/src/ros2/rcl/rcl/src/rcl/node.c:92

Please restore the proxy object stuff or check that rclpy.init() has been called in each of the cases and raise the right exception.

This is the code that checked on each access of _rclpy and raise if init() had not been called:

https://github.com/ros2/rclpy/pull/82/files#diff-7486716035fb1f918746b7f5b1b31b35L49

[wjwwood]

I created #84 to track the issue since this cannot be reopened.

[wjwwood]

Also, after this pr, NoImplementationAvailableException is no longer used anywhere. I'll remove it in my upcoming pr.