Made error messages a bit more meaningful; also, only output them it there was an actual error
Removed development service from Gazebo plugin
Nicified output
Made sure that plugins are only loaded once (based on their *real* name)
Remove entries from the list of plugins to load before loading a new config file

In case a config file has begun to be loaded, and threw an exception, already existing entries in the list of plugins to load would have survived this (and, therefore, would be loaded when the next successful config file parse was done). This is fixed now.
Nicified output of ROS plugin