Automatic download: fail when URL is wrong using curl

[dyfer]

We want curl to return non-0 exit code when the URL points to a non-available resource

Fixes #116

[joslloand]

@dyfer merged into dev branch. I'll make a new minor release, which also contains an update (752998e193219381266c060e81f6589b7a2fa310) from @mtmccrea