Backport PR
#2544: Infinite loop when multiple debuggers have been attached....
Backport PR
#2544: Infinite loop when multiple debuggers have been attached.
Replaces pull-request
#2388.
Fixed infinite loop on exit in the event of where multiple debuggers have been attached an there is an uncaught exception.
Test code:
```python
from IPython.core.debugger import Tracer;
Tracer()
Tracer()
raise Exception
```
Pinging @DamianHeard to review.