Gemspec in not base directory causes silent instillation error

When I had the gemspec file in a directory that wasn't the base directory, bin/logstash-plugin install would state succeed but the plugin would not show up in the list.

Moving this to the base directory and fixing the gemfile then allowed for the plugin to be properly installed.

* It would be nice to see an error when the plugin isnt actually installed
* It would be nice for support for the gemspec outside the base dir.

For all general issues, please provide the following details for fast resolution:

- Version: logstash-5.4.1-1.noarch
- Operating System: Centos 7.2
- Config File (if you have sensitive info, please remove it):
- Sample Data:
- Steps to Reproduce:
In logstash-filter-xxx.gemspec
` s.files = Dir['lib/**/*','spec/**/*','vendor/**/*','directory/*.gemspec','*.md','Gemfile'] `
And then build the gem, and install.
```
bin/logstash-plugin --local --no-verify logstash-filter-xxx.gem
Installing logstash-filter-xxx
Installation successful
bin/logstash-plugin list | logstash-filter-xxx
```