Exception handling in BackendPlugin has been improved by adding
user-friendly error messages for various exceptions. Errors during
backend plugin start or stop will now trigger a message to the user,
providing more context about the cause of the failure. This makes it
easier for users to understand and resolve possible issues. In addition,
the EngineBackend stop function has been modified to forcibly stop all
running backend plugins instead of allowing multiple plugins to run
simultaneously.
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