Don't throw IOError for missing .env file

[lachlancooper]

First of all thanks very much for this beautiful library, I'm using it to replace a few others as well as a lot of manual type-casting.

I would like to suggest making read_env() only opportunistically search for a .env file, and do nothing (or perhaps log a warning) if no file is found. This is the way django-environ, envparse, and python-dotenv all work.

I think it makes sense to support .env files in situations where there is no better way to manage environment variables, such as running an application locally, but not require them when running in production under, say, Docker or a traditional process manager.

I can of course work around this with a simple try/catch, but a change like this would make the library even nicer to use.

[sloria]

Thanks for the kind words.

I'm open to this suggestion. If it's the convention to pass silently, then we should probably do the same.

PR's welcome! =)

[euphoria]

I don't know whether it is convention or not, so I don't want to impose it on this library without knowing, but for what it's worth I manually use

try:
    env.read_env(recurse=True)
except IOError:
    pass

myself as well.

[sloria]

In 3.0.0, a warning is raised instead of an error.