Report if tracing code causes an error

[fikovnik]

If the code that is running in the call_exit_hook causes an error, I still get:

List of 2
 $ result:List of 2
  ..$ error:Class 'instrumentr_undefined' <environment: 0x55660a593a18> 
  ..$ value: NULL
  ..- attr(*, "class")= chr "instrumentr_result"
 $ data  :<environment: 0x55660c7d4108> 

as the result. Moreover, after the error, no more hooks are called.