[#334] Expect custom contract errors in nettest

Description

Problem: Currently it's very inconvenient to expect contract failure in nettest scenario when contract fails with custom defined error.

Solution: Add expectCustomError and expectCustomError_ actions to the nettest which are similar to lExpectCustomError and lExpectCustomError_ from Lorentz.Test.Integrational.

Related issue(s)

Resolves #334 (closed)

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