Report conflicted OTA requests

[rjtokenring]

Motivation

In case an OTA cannot be started due to a conflict (an OTA is already in progress), report it.

Reviewer checklist

• [ ] PR address a single concern.
• [ ] PR title and description are properly filled.
• [ ] Changes will be merged in main.
• [ ] Changes are covered by tests.
• [ ] Logging is meaningful in case of troubleshooting.
• [ ] History is clean, commit messages are meaningful (see CONTRIBUTING.md) and are well formatted.