Give better feedback for install/update when rubygems is unreacheable

This situation mostly happen when:
- rubygems is really down
- You don't have direct access to rubygems
- The `HTTP_PROXY` is misconfigured, or the host doesn't proxy the request.

When any above scenario occurs, the plugin manager will return a non-helpful message like this.

```
Error Bundler::InstallError, retrying 1/10
An error occurred while installing logstash-core-event-java (5.0.0.pre.rc1), and Bundler cannot continue.
Make sure that `gem install logstash-core-event-java -v '5.0.0.pre.rc1'` succeeds before bundling.
WARNING: SSLSocket#session= is not supported
Error Bundler::InstallError, retrying 2/10
An error occurred while installing logstash-core-event-java (5.0.0.pre.rc1), and Bundler cannot continue.
Make sure that `gem install logstash-core-event-java -v '5.0.0.pre.rc1'` succeeds before bundling.
WARNING: SSLSocket#session= is not supported
```

When we are not doing an `--local` install, we should try to connect to rubygems with the appropriate proxy or not and skip the retry loop if we cannot connect.

Related to https://github.com/elastic/logstash/issues/6044