Raise exception for invalid s3 credentials

[sobrinho]

See refile/refile#454

[sobrinho]

@jnicklas see if that's the way to go.

Basically we need better error messages due to typos instead of strange exceptions.

Does makes sense?

[klausbadelt]

AWS credentials as options should be actually considered optional (see other pull requests like #29 #18. Recommended on EC2 is actually instance profiles. See also AWS SDK configuration.

[sobrinho]

So, maybe in that case we could create a special value like Refile::S3::NO_VALUE, I don't know.

What you suggest?

We must have better exception messages.

[filmhubhq]

Doesn't AWS SDK already raise exceptions for missing configuration?

[sobrinho]

Not sure but on refile/refile#454 you will see that the error appears on refile itself, not on SDK.

[sobrinho]

Closing, if we found a better way to do that, a new pull request will be done.