Series sales form: validate price

[php-coder]

Steps to reproduce:

• login as admin
• open page with series info
• in the series sales form set Price (or Alternative price) to -1 (or 0)
• submit form

Current result:

• Price (or Alternative price) field doesn't show error

Expected result:

• Price (or Alternative price) field should show error

Details:

• We can use @Price validator for that
• The similar test case that can be copy&pasted: WhenUserAddSeries.catalogPricesShouldRejectInvalidValues

Tech debt for: fe9f30d (#198)

[php-coder]

@Tachkin I've updated issue's description to provide more detailed explanation. Feel free to ask if you have any questions.