FIX: Output of the ERROR message of const_get method is not enough.

This PR is fix my PR #54.

If initializer of Fog::Identity::ProviderName in line 17 raises an exception,
the exception is caught by `rescue` block and eventually `NameError` will be raised in line 19.
This hides the internal exception and makes debugging hard.

```ruby
class Fog::Identity::ProviderName
  class Real
    def initialize
        raise "error"
    end
  end
end

# No definition of Fog::ProviderName::Identity
```