Add initial Platform support for reporting discovery issues

[marcphilipp]

Overview

This PR adds DiscoveryIssue along with a new issueEncountered method on EngineDiscoveryListener. The launcher reports engines with issues of severity ERROR as failed during execution. Which severity is considered "critical" will be made configurable in a later PR.

Issue: #242

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes will be done in a later PR