rviz display crash when layer is not available

[bmagyar]

GridMap throws exceptions when lookup of things fail but these are not caught in the rviz display which in return crashes rviz altogether.

terminate called after throwing an instance of 'std::out_of_range'
  what():  GridMap::at(...) : No map layer 'elevation' available.

To reproduce:

• add new display
• set "Height Layer" to something invalid
• change "Height Transformer" to "Flat"
• start publishing gridmaps