assert shall not be used in production code (control flow)

[boris-42]

Asserts should be used only in tests, and should not be used in production code, like here and in other places

https://github.com/RedisGraph/redisgraph-py/blob/master/redisgraph/path.py#L7

There are 2 problems:

1. If you run with python -o, all these asserts will be ignored and code will stop working
2. It's a best practice to use own Exceptions or at least default like ValueError with human readable messages