Export types; correct error prototype

[FabijanC]

Usage related changes

• Export types
• Fix prototype of StarknetPluginError

Development related changes

• Will allow better testing

Checklist:

• Formatted the code
• No linter errors + tried to avoid introducing linter warnings
• Performed a self-review of the code
• Rebased to the last commit of the target branch (or merged it into my branch)
• Documented the changes
• All tests are passing (for external contributors who don't have access to the CI/CD pipeline)