carmel install should exit with non-zero when dependencies are not satisfied

[miyagawa]

> carmel install
---> Installing new dependencies: Plack, Plack::Request

gzip: stdin: not in gzip format
/bin/tar: Child returned status 1
/bin/tar: Error is not recoverable: exiting now
! Failed to fetch distribution Plack-1.0041
! Installing the dependencies failed: Module 'Plack' is not installed, Module 'Plack::Request' is not installed
! Bailing out the installation for ..
Can't find an artifact for Plack => 1.0041
You need to run `carmel install` first to get the modules installed and artifacts built.

> echo $?
0