Add check that environment is properly set up for FogROS before launch

[jeffi]

Describe the bug When launching FogROS, it's possible to get far into the launch before observing an error that could have been caught early. For example, setting RMW_IMPLEMENTATION incorrectly (not to the supported rmw_cyclonedds_cpp) or setting CYCLONEDDS_URI to point to an incompatible configuration file.

To Reproduce Steps to reproduce the behavior:

1. Make sure RMW_IMPLEMENTATION and CYCLONEDDS_URI are both not set
2. Launch the talker example
3. Observe that the listener and talker either cannot communicate, or there may be an exception.

Expected behavior At launch, before creating an instance, produce an error and fail if the environment variables are incorrectly set.

[KDharmarajanDev]

Resolved by #59