lti_tool_provider_goto wrapper should function like the thing is wrapping

lti_tool_provider_goto is used in place of drupal_goto inside the module, however it does not behave similarly to the outside world - it returns! Callers of drupal_goto expect the function to never return, and so do the functions inside the module.

All paths should be finished with drupal_exit() instead of a return, leaving only the valid path to really goto drupal_goto().