This removes the use of RuntimeError, and replaces it with a custom
error class called ZulipError. In a few places, we use a subclass to
make it easier for code to interact with the error type.
As the first unittest, this creates a test directory and
abnd adds it tho the excluded pip package files.
There are two `tests` directories now, one in zulip_botserver and one in
zulip. This confuses the unittest runner, leading to failed test imports.
Therefore, we need to tell the package importer that there are multiple
tests directories, all of which should be considered for a search.
