Installing empty plugins

If you install (with `bin/plugin` ) an empty, aka no `*.gemspec`, plugin LS does not complain during the installation process, however afterwards `bin/plugin list` does not show the gem content wise it was not good enoughs.

details:

```
╭─purbon@purbon-elastic ~/work/logstash ‹feature/plugins_offline›
╰─$ bin/plugin install /Users/purbon/Downloads/antonio/logstash-filter-csvlookup-test/logstash-filter-csvlookup-test-0.1.4.gem
Validating /Users/purbon/Downloads/antonio/logstash-filter-csvlookup-test/logstash-filter-csvlookup-test-0.1.4.gem
Installing logstash-filter-csvlookup-test
Installation successful
╭─purbon@purbon-elastic ~/work/logstash ‹feature/plugins_offline*›
╰─$ cat Gemfile
# This is a Logstash generated Gemfile.
# If you modify this file manually all comments and formatting will be lost.

source "https://rubygems.org"
gem "logstash-core", "2.0.0.dev", :path => "."
gem "file-dependencies", "0.1.6"
gem "ci_reporter_rspec", "1.0.0", :group => :development
gem "simplecov", :group => :development
gem "coveralls", :group => :development
gem "rspec", "~> 3.1.0", :group => :development
gem "logstash-devutils", "~> 0", :group => :development
gem "octokit", "3.8.0", :group => :build
gem "stud", "~> 0.0.19", :group => :build
gem "fpm", "~> 1.3.3", :group => :build
gem "gems", "~> 0.8.3", :group => :build
gem "logstash-filter-csvlookup-test", "0.1.4", :path => "vendor/local_gems/d117c477/logstash-filter-csvlookup-test-0.1.4"
╭─purbon@purbon-elastic ~/work/logstash ‹feature/plugins_offline*›
╰─$ bin/plugin list --verbose
ERROR: No plugins found
╭─purbon@purbon-elastic ~/work/logstash ‹feature/plugins_offline*›
╰─$ bin/plugin list 1 ↵
ERROR: No plugins found
╭─purbon@purbon-elastic ~/work/logstash ‹feature/plugins_offline*›
╰─$ ls vendor/local_gems/d117c477/logstash-filter-csvlookup-test-0.1.4 1 ↵
╭─purbon@purbon-elastic ~/work/logstash ‹feature/plugins_offline*›
╰─$ cd vendor/local_gems/d117c477/logstash-filter-csvlookup-test-0.1.4
╭─purbon@purbon-elastic ~/work/logstash/vendor/local_gems/d117c477/logstash-filter-csvlookup-test-0.1.4 ‹feature/plugins_offline*›
╰─$ ls
╭─purbon@purbon-elastic ~/work/logstash/vendor/local_gems/d117c477/logstash-filter-csvlookup-test-0.1.4 ‹feature/plugins_offline*›
╰─$ ls -la
```

Even rare, this situation could happen, I think we should detect them somehow and react as expected, for example reporting the error to the user and aborting the installation.