Provide try_register test utility

[dmkozh]

What problem does your feature solve?

Currently the register function unwraps the return value for the contract instantiation, which makes testing the constructor errors impossible.

What would you like to see?

Add a try_register function that returns a Result and thus allows testing the constructor error conditions. Env already returns a Result from the respective functions.

What alternatives are there?

It's possible to cover constructor errors using a test-only deployer contract, but that is more cumbersome and only works with Wasm contracts.