Improve error message handling for spdlog

[christophebedard]

Kind of relates to ros2/rcutils#269

1. Modify tests to expect the error message to be set when it should be set and then reset it
2. In rcl_logging_spdlog's rcl_logging_external_initialize(): in case of rcl_logging_get_logging_directory() failure, append error message instead of overwriting it without resetting it (and thus generating a "This error state is being overwritten" warning)
   • The error message looks like:

     Failed to get logging directory: rcutils_join_path failed, at /home/user/ros2_ws/src/ros2/rcl_logging/rcl_logging_interface/src/logging_dir.c:59, at /home/user/ros2_ws/src/ros2/rcl_logging/rcl_logging_spdlog/src/rcl_logging_spdlog.cpp:136
     

3. Document that rcl_logging_external_initialize() must/should(?) set an error message if it returns RCL_LOGGING_RET_ERROR

This results in rcl_logging_spdlog/test/test_logging_interface.cpp not generating any "This error state is being overwritten" warnings.

[christophebedard]

This is a draft pending higher-level discussion for overall error message handling.

[clalancette]

In short, this seems reasonable to me. I'm going to hold off on approving since this is still a draft and we need to have the higher-level discussion.

[fujitatomoya]

draft looks good to me