Validate file names and path names #15
Description
Find a good and general way to test the validity of variable names.
E.g. client.create_dataset("/this/is/a/path") should produce a 422 NamingError because we don't want documents which are named "/this/is/a/path"
but it actually produces a 404 DataNonExistentError.
The functions in galahad/server/util should filter this case out.