Error messages in the new promotion system are hard to understand

[fthobe]

If a new code is created while the same code is already present as draft, the following error is displayed:
"Codes is invalid"

1. "Codes" is always plural even if only one code is created
2. "Code(s) are / is already present in the System" would be better

Solidus Version:
4.4.2

To Reproduce

1. create an incomplete promotion
2. try to create a new promotion with the same code