Improve settings currency prevent user error.

| Q	                       | A     |
|--------------------------|------ |
| Bug Fix?        	| n     |
| New Feature?	| n     |
| BC Breaks?	       | n     |
| Deprecations?	| n     |
| Tests Pass?	       | n     |
| Fixed Tickets	       |        |
| License	               | MIT |
| Doc PR                  |        |

This PR prevent user error, if user select none existing currency Sylius will throw error in many pages.
@liverbool Is there spec for this class ?