No error message when custom buildpack download fails

[jarthod]

When passing an invalid buildpack URL like a simple string "ruby", the download fails without any error or explanation, when building it finishes like this:

> dokku buildpacks:add suzaku ruby
> dokku buildpacks:list suzaku
-----> suzaku buildpack urls
       ruby
> dokku ps:rebuild suzaku
-----> Cleaning up...
-----> Building suzaku from herokuish...
-----> Adding BUILD_ENV to build environment...
-----> Warning: Multiple default buildpacks reported the ability to handle this app. The first buildpack in the list below will be used.
       Detected buildpacks: multi ruby
-----> Multipack app detected
=====> Downloading Buildpack: ruby

→ the output stops here without any error, it's not even clear it failed in this case.

I also made one ticket about validating the buildpack URL: dokku/dokku#3923
But even so some error handling need to be implemented here as the download could fail for other reasons not possible to validate when adding the URL.