Fix error message to include all errors in Spree::Order instance

* When Spree::Order#next fails because an association is invalid, a
  red blank error message is displayed in the cart. This hides
  legitimate errors, like a bug I found in spreee_active_shipping.

Fixes #3714
Fix order_id assignment in OrderInventory

We missed the order param when creating inventory units through
shipments in OrderInventory. That will cause exception later on because
a InventoryUnit expect to always be associated with an order (which is
something I believe we should probably remove completely in the next
major release)

Conflicts:
	core/app/models/spree/order_inventory.rb

Fixes #3713
Remove blank username line in dummy database.yml mysql config

For some reason activerecord-jdbcmysql-adapter hates blank usernames
and will not connect despite the fact that mysql2 works fine. Removing
these lines doesn't break mysql2 so I see no reason to keep them there.
I'm trying to test my gems on jruby in anticipation of spree working on
jruby so this fix would be helpful. Here's sample Travis output showing
what goes wrong:
https://travis-ci.org/swrobel/spree_zero_stock_products/jobs/11136157

Fixes #3693
Merge branch 'source/2-0-stable' into 2-0-stable

# By Dan Kubb (1) and others
# Via Ryan Bigg
* source/2-0-stable:
  Remove blank username line in dummy database.yml mysql config
  Fix order_id assignment in OrderInventory
  Fix error message to include all errors in Spree::Order instance