Improve error handling from FFI errors

[ghost]

Purpose

The CCD scan currently uses string comparison to handle errors from FFI calls. Due to PR Concordium/concordium-net-sdk#93 on the .NET SDK it is now possible to match on exact error codes.

Task: https://concordium.atlassian.net/browse/CBW-1662

Changes

• Update handling of errors from FFI calls to use error codes

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.