Log plugin registration/load failures instead of printing (PR #2243)

[eduralph]

Non-Mantis gramps contribution — tracked as a fork issue per the no-Mantis convention.

• Upstream PR: Log plugin registration and load failures instead of printing gramps-project/gramps#2243
• Branch: plugin-improvements/structured-errors

No Mantis ticket exists for this item, so this issue is its tracking entry. Close when the PR merges or is dropped.