ENH: Accept one-off exception mode setting for %tb....
ENH: Accept one-off exception mode setting for %tb.
This makes it easy to access a more verbose representation that the
currently-active exception reporting mode provides.
```python
In [1]: %xmode minimal
Exception reporting mode: Minimal
In [2]: def f():
...: 3/0
...:
In [3]: f()
ZeroDivisionError: division by zero
In [4]: %tb verbose
---------------------------------------------------------------------------
ZeroDivisionError Traceback (most recent call last)
<ipython-input-3-c43e34e6d405> in <module>
----> 1 f()
global f = <function f at 0x10e52de18>
<ipython-input-2-cd4172d70b5b> in f()
1 def f():
----> 2 3/0
3
ZeroDivisionError: division by zero
In [5]: f() # The default reporting mode has not been changed....
ZeroDivisionError: division by zero
```
