Better exception reporting when --pgtap-uri or DATABASE_URL is missing

[lmergner]

Better usage error when DATABASE_URL is not set...

ImportError while loading conftest '/Users/lmerg/code/pestle/tests/conftest.py'.
tests/conftest.py:11: in <module>
    engine = create_engine(DATABASE_URL)
../../.pyenv/versions/3.7.3/envs/test_pestle/lib/python3.7/site-packages/sqlalchemy/engine/__init__.py:435: in create_engine
    return strategy.create(*args, **kwargs)
../../.pyenv/versions/3.7.3/envs/test_pestle/lib/python3.7/site-packages/sqlalchemy/engine/strategies.py:56: in create
    plugins = u._instantiate_plugins(kwargs)
E   AttributeError: 'NoneType' object has no attribute '_instantiate_plugins'