Traceback improvements

Previously, error tracebacks for errors happening in code cells were showing a hash, the one used for compiling the Python AST:

In [1]: def foo():
...:     return 3 / 0
...:

In [2]: foo()
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-2-c19b6d9633cf> in <module>
----> 1 foo()

<ipython-input-1-1595a74c32d5> in foo()
    1 def foo():
----> 2     return 3 / 0
    3

ZeroDivisionError: division by zero

The error traceback is now correctly formatted, showing the cell number in which the error happened:

In [1]: def foo():
...:     return 3 / 0
...:

In [2]: foo()
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
In [2], in <module>
----> 1 foo()

In [1], in foo()
    1 def foo():
----> 2     return 3 / 0

ZeroDivisionError: division by zero