catch non-existing convex pools gracefully

[gosuto-inzasheru]

there should be a check in withdraw_all to make sure the passed asset has a pool.

current behaviour is a bit ugly:

>>> safe.convex.withdraw_all(usdc)
  File "<console>", line 1, in <module>
  File "./great_ape_safe/api_safe/convex.py", line 75, in withdraw_all
    pool_id = self.get_pool_info(underlying)[0]
TypeError: 'NoneType' object is not subscriptable

i am sure other functions have similar issues, maybe a helper function (eg convex._is_valid_underlying(...)) makes sense here.