bugfix: when no regions are loaded, loading of a region with subregions fails e.g. loading :de_bb fails

The following bug occurs if the first region you want to load is a region with subregion, e.g. :de_bb

```
irb(main):001:0> require "holidays"
=> true
irb(main):002:0> Date.today.holiday? :de_bb
Holidays::UnknownRegionError: Could not load holidays/de_bb
irb(main):003:0> Date.today.holiday? :de
=> false
irb(main):004:0> Date.today.holiday? :de_bb
=> false
```

Unfortunately there are regions in lib/holidays with "_" in their name. This is bad because this symbol is also used to specifiy subregions. Therefore i had to nest two rescue blocks in my commit and try to load the prefix. 

Ps: This does not raise an error if you require :de_bbd (which does not exist) but that is another issue which already existed in the previous version.