serverIsActive to is_server_active

catch exceptions by reference and print relevent information

* changed error message

* changed the error message

* forgot to add return after the error message

* fixed the coding sytle. It should pass the linter test now

Without these changes, I've been seeing lots of 'foo has already been
declared' errors.

revert dwb_critics cmake changes

adding test cases

move collision checker to nav2_costmap_2d

* update bt navigator readme