go: plugin_proxy: error on plugin initialization errors

[stoksc]

When a Go plugin fails in initialization, with this change, it crashes fluentbit instead of only printing an error log. Also, proxy_go_input_init no longer frees memory that is also cleaned up on flb exit (e.g., a return from flb_input_thread.c:flb_input_thread_instance_init:610 sees a double free, even before this change).

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Testing
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