Use get() rather than a try-catch block in memoized function lookup.
Currently backtraces often look like this:
```
---------------------------------------------------------------------------
KeyError Traceback (most recent call last)
~/p/jax/jax/util.py in memoized_fun(*args, **kwargs)
133 try:
--> 134 return cache[key]
135 except KeyError:
KeyError: ((lu, ShapedArray(int32[2,2])), ())
During handling of the above exception, another exception occurred:
KeyError Traceback (most recent call last)
~/p/jax/jax/util.py in memoized_fun(*args, **kwargs)
133 try:
--> 134 return cache[key]
135 except KeyError:
KeyError: ((lu, xla_client.Shape(_dtype=dtype('int32'), _dimensions=(2, 2), _is_tuple=False, _minor_to_major=None)), ())
During handling of the above exception, another exception occurred:
NotImplementedError Traceback (most recent call last)
<ipython-input-26-d6c00d50e3c9> in <module>
```
The "during handling of the above exception..." message is mostly a distraction for the user that occurs because we perform the memoized function evaluation inside a `catch` block. By performing the function evaluation outside the catch block, we can get better backtraces without the distraction of the KeyError exception.
```
jax
8127392a - Use get() rather than a try-catch block in memoized function lookup.
