fail if specified .env file does not exist

[rptaylor]

Previously discussed in #199

See theskumar/python-dotenv#297

It looks like now there is https://github.com/theskumar/python-dotenv/pull/388/files which will cause load_dotenv to return false if no vars were read:

import dotenv
a=dotenv.load_dotenv('no.env', verbose=True)
print(a)
False

So for the reasons previously described it would be good to have a way in environs to force a failure if the specified env file does not exist, by e.g. checking the return code from dotenv and raising an exception if it is false.

[sloria]

#311 modifies env.read_env to return False if the file is not found. This allows users to handle that case as they see fit, e.g. raise an exception, log a warning.

Closing this for now. Thanks for the suggestion!

[rptaylor]

Great , thanks @sloria !