Explicit support for importing remote CSVs

It worked before, but threw a warning that the file was missing.
Throw an explicit warning when the file doesn't exist

The iterator will eventually error anyway. Let's do this before we get to that.
Make this error more explicit by including response code
Add tests for error case
Test case for remote csv error
Merge pull request #735 from wp-cli/remote-import-csv

`wp user import-csv` should support remote files