Make error message explicit

[tarekziade]

https://github.com/mozilla-services/cliquet/blob/master/cliquet/storage/postgresql/client.py#L61

"PostgreSQL dependencies missing" is not very clear.

Should be "SQLAlchemy missing" maybe ?

[Natim]

Actually the way to install cliquet postgresql dependencies is to use:

pip install cliquet[postgresql]

rather than just installing SQLAlchemy.

[tarekziade]

@Natim this is unrelated to the issue

[leplatrem]

Would you prefer a link to docs in the error message? Or suggesting the command to run?

Or Python libraries for PostgreSQL are missing ?

[tarekziade]

I would explicitly indicate in the error which dependency is missing or has a problem.

message = ("PostgreSQL sqlalchemy dependency missing. "
                   "Refer to installation section in documentation.")

The reason is that sometimes you can have an installation error and knowing which lib is screwed can be useful.

[leplatrem]

We wanted to avoid mentioning it explicitly, but if it can make sense, why not!

As @Natim said there are several ones, but I think that if some of the other ones are missing, it fails somewhereelse within sqlalchemy. So why not mentionning sqla here indeed...

[tarekziade]

As @Natim said there are several ones

yes, in this code, I see "if sqlachemy is None:"