Assert specific error codes in failure tests

[Glittersup]

Summary

Several failure tests assert only that a call failed, not which error was returned. That means a change that swaps one error for another still passes, even though it is a breaking interface change for any client mapping those codes.

Acceptance criteria

• Failure tests assert the specific StreamError variant returned.
• No failure test asserts only that an error occurred.
• The suite still passes.

Getting started

Fork this repository, clone your fork, and add this repo as upstream:

git clone https://github.com/<your-username>/tricklepay-contracts.git
cd tricklepay-contracts
git remote add upstream https://github.com/TricklePay/tricklepay-contracts.git

Create a branch for this issue:

git checkout -b test/issue-176

Suggested commit message:

test: assert specific error codes

Run cargo fmt --check, cargo clippy --all-targets -- -D warnings, and cargo test before opening a pull request and linking this issue.