show gem name when throwing GemNotInHomeException

The original error is not very useful:
```bash
ERROR:  Gem::GemNotInHomeException: Gem is not installed in directory /home/mpapis/.rvm/gems/ruby-2.1.0-preview2
```
after fix:
```bash
ERROR:  Gem::GemNotInHomeException: Gem 'rake-10.1.0' is not installed in directory /home/mpapis/.rvm/gems/ruby-2.1.0-preview2
```