Provide clear error message if dependency is not available

[chris-rock]

Description

If I run InSpec on a profile with multiple dependencies, it aborts if a dependency is not available. This is correct, but its hard for the user to extract the abortion from No fetcher for linux

$ inspec exec examples/meta-profile 
[2016-09-14T08:47:39+02:00] WARN: URL target https://github.com/dev-sec/tests-ssh-hardening transformed to https://github.com/dev-sec/tests-ssh-hardening/archive/master.tar.gz. Consider using the git fetcher
[2016-09-14T08:47:42+02:00] WARN: URL target https://github.com/dev-sec/tests-ssh-hardening transformed to https://github.com/dev-sec/tests-ssh-hardening/archive/master.tar.gz. Consider using the git fetcher
No fetcher for linux (options: {:cwd=>"examples/meta-profile", :backend=>#<#<Class:0x007fc2cd5ca8e0>:0x007fc2cd5c3180>, :name=>"linux", :compliance=>"base/linux"})

InSpec and Platform Version

0.34.0

Replication Case

see above

Possible Solutions

Show proper error message and color it in red.

Related to #888

[stevendanna]

I believe this was fixed in: #1126