ZEO/Blob support: superfluous file existence test

[Christian Theune]

In ZEO.ClientStorage around line 1035 there is a test that after `server.sendBlob` the file exists, otherwise an exception is thrown.

We consider this to be superfluous, because:

- if the file could not be sent then `sendBlob` should have raised an exception
- the test whether the expected file exists therefore only covers the possibility
  that someone outside the system removed the file. However, this test and the `return`
  are not atomic and therefore the test doesn't really help.

Our point is somewhat supported by the fact that no test exists for the code path where
the exception would be raised. This in turn currently hides a NameError.

[Christian Theune (ctheune)]

Committed in r83744.

[Christian Theune (ctheune)]

I'll port the fix soon.