Preserve original error in handleFailure for monitoring and logging

[Copilot]

The handleFailure function was overwriting the original error with the failure handler's return value, causing monitoring and logging to report incorrect errors when handlers returned nil or different errors.

Changes

• Preserve original error in originalErr variable before calling failure handler
• Capture failure handler's error return in separate handlerErr variable
• Use originalErr for ReportRequestFailure and final failure log
• Log failure handler errors separately when present

Example

Before:

if fexec.flow.FailureHandler != nil {
    data, err = fexec.flow.FailureHandler(err)  // Overwrites original err
}
// Later: monitoring and logging use potentially modified err
fexec.eventHandler.ReportRequestFailure(fexec.id, err)

After:

originalErr := err
if fexec.flow.FailureHandler != nil {
    var handlerErr error
    data, handlerErr = fexec.flow.FailureHandler(originalErr)
    if handlerErr != nil {
        fexec.log("[request `%s`] failure handler returned error: %v\n", fexec.id, handlerErr)
    }
}
// Monitoring and logging use original error
fexec.eventHandler.ReportRequestFailure(fexec.id, originalErr)

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