Misspelled plugin name causes confusing exception

[solnic]

Describe the bug

When a plugin name is misspelled, you'll end up with a confusing exception.

To Reproduce

class MyContainer < Dry::System::Container
  use :roargh
end

Results in:

NoMethodError:
  undefined method `load_dependencies' for nil:NilClass
# .gem/ruby/2.4.1/gems/dry-system-0.12.0/lib/dry/system/plugins.rb:86:in `use'

Expected behavior

A meaningful error should be raised saying "Plugin :roargh does not exist"