Throw SkipRowException in CommercePrice process plugin when there is no currency_code

It looks like part of the price object isn't always added. This patch looks up the currency related decimal places and adds it to the price. It also fixes one of the tests.

I added this patch to the related issue #3262612 based on some of the comments there but there are other issues wrapped up there too. I think this solves this particular problem.

Trying for a title that expresses the change.

Or, if there is no currency code should we get the default current code? See #3284861-6: commerce1 payment transaction with empty currency_code crashes migrate → for more on this idea.

I think this is ready to go.

Automatically closed - issue fixed for 2 weeks with no activity.