New error handling for lack of golem.com.payment.chosen-platform

[golmek]

Summary:

When using Golem Core Python (https://github.com/golemfactory/golem-core-python) Rinkeby is used as a payment network even tough mainnet was explicitly chosen. This is due to lack of golem.com.payment.chosen-platform property in Golem Core Python and will be fixed in: golemfactory/golem-core-python#18

However, current behaviour of Yagna is prone to errors as we accept lack of golem.com.payment.chosen-platform parameter and then choose default -> Rinkeby. This results in hard to debug problems when testnets are not stable or getting deprecated (like Rinkeby). Those impact projects like Ray on Golem & Jupyter on Golem.

New approach should require this parameter and throw error when it is missing. No default path.

Acceptance Criteria:

• Lack of golem.com.payment.chosen-platform should throw explicit error.
• Disable default path for situation when there is no golem.com.payment.chosen-platform

Additional Information:

• Ticket Related to: Rinkeby invoices when mainnet invoices are expected #2272