Get error string from rcl layer

[nnmm]

Storing the error string requires a new struct, which I've named RclrsError.

This also makes Display impls use a period instead of an exclamation mark.

Closes #111

[nnmm]

As an example (I added .context("Failed to create context") and .context("Failed to do awesome thing") in appropriate places in the minimal subscriber, so the chain is longer than just two entries):

cargo run --bin minimal_subscriber -- --ros-args -p plem
   Compiling rclrs_examples v0.2.0 (/home/user/workspace/src/ros2_rust/rclrs_examples)
    Finished dev [unoptimized + debuginfo] target(s) in 2.72s
     Running `target/debug/minimal_subscriber --ros-args -p plem`
[ERROR] [1652473951.278531464] [rcl]: Failed to parse global arguments
Error: Failed to do awesome thing

Caused by:
    0: Failed to create context
    1: RclReturnCode::ParsingError: Found invalid ROS argument while parsing.
    2: Couldn't parse parameter override rule: '-p plem'. Error: Expected lexeme type (22) not found, search ended at index 4, at /tmp/binarydeb/ros-galactic-rcl-3.1.2/src/rcl/lexer_lookahead.c:233

2 is the error string from the rcl layer.