Added the test for the import error.

[gabrieldemarmiesse]

We have an issue with error messages for optional dependencies. For exemple, trying to use the sql observer without sql installed throws the following error:

tests/test_observers/test_sql_observer_not_installed.py:11 (test_importerror_sql)
ex = <sacred.experiment.Experiment object at 0x7f5495d9f4a8>

    @pytest.mark.skipif(has_sqlalchemy, reason='We are testing the import error.')
    def test_importerror_sql(ex):
        with pytest.raises(ImportError):
>           ex.observers.append(SqlObserver.create('some_uri'))

test_sql_observer_not_installed.py:15: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

cls = <class 'sacred.observers.sql.SqlObserver'>, url = 'some_uri', echo = False
priority = 40

    @classmethod
    def create(cls, url, echo=False, priority=DEFAULT_SQL_PRIORITY):
>       engine = sa.create_engine(url, echo=echo)
E       NameError: name 'sa' is not defined

../../sacred/observers/sql.py:23: NameError

Not very helpful. See here the travis log for the test I added.

This PR makes sure that we have a proper error message.

I had to enable the test for the observers in the "setup" build. Otherwise the added test will never run.

With the new setup, the error message is:

Traceback (most recent call last):
  File "/mnt/c/Users/gdemarmi/Desktop/projects/sacred/trying_stuff.py", line 9, in <module>
    ex.observers.append(SqlObserver.create('some_uri'))
  File "/mnt/c/Users/gdemarmi/Desktop/projects/sacred/sacred/observers/sql.py", line 19, in create
    from sqlalchemy.orm import sessionmaker, scoped_session
ModuleNotFoundError: No module named 'sqlalchemy'

The majority of the change is just a copy past between sql.py and sql_bases.py.

[coveralls]

Coverage decreased (-0.2%) to 85.305% when pulling a0e801f on gabrieldemarmiesse:better_import_message into 7054628 on IDSIA:master.

[JarnoRFB]

Is there any reason you are import sqlalchemy here from sql_bases instead of directly?

[gabrieldemarmiesse]

Yes, there is a very good reason. It's because I wasn't attentive, haha.

[JarnoRFB]

Haha ok. PR looks good, I will merge once the tests pass after the change.