Display error message instead of stack trace when trying to update plugins with wrong privilege.

I noticed that when not having the permission to update shows something like this:

```
/opt/logstash$ bin/plugin update logstash-input-file
Errno::EACCES: Permission denied - /opt/logstash/Gemfile
initialize at org/jruby/RubyFile.java:363
new at org/jruby/RubyIO.java:853
gemfile at /opt/logstash/lib/pluginmanager/command.rb:3
execute at /opt/logstash/lib/pluginmanager/update.rb:12
run at /opt/logstash/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:67
execute at /opt/logstash/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/subcommand/execution.rb:11
run at /opt/logstash/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:67
run at /opt/logstash/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:132
(root) at /opt/logstash/lib/pluginmanager/main.rb:36
```

Elevating my privileges fixed it. However, it would be nice to have an error message instead of this.