Raise not exists error when attempting to get size for unexisting S3 file

[sevdog]

As of now, attempting to get the file size from S3Storage always raises a botocore.ClientError, which is vague and may not fall under existing file-management related error handling.

Since may other methods in S3 storage already implement this kind of logic I have put the same in the S3Storage.size method. Now if the S3 error is a "not found" a more explanatory FileNotFoundError is raised instead.

[jschneier]

Closes #1022

[jschneier]

Thanks