Make specific transmission interfaces proper types.

- Proper types instead of namespaces allow to provide less cryptic feedback.
  * Using typedefs:
  "transmission_interface::TransmissionInterface<transmission_interface::ActuatorToJointPositionHandle>"

  * Using a new type:
  "transmission_interface::ActuatorToJointPositionInterface"

- Added error message printing to tests for manual inspection.
Merge pull request #70 from pal-robotics/master

Make specific transmission interfaces proper types.