IME: Add convenience `is_ok()` method on error type #57

Job	Run time
Code Quality	12s
Build (stable)	13s
Build with API feature levels (api-10)	13s
Build (beta)	12s
Build with minimal features (api-10, drawing)	12s
Build with API feature levels (api-11)	13s
Build Documentation	12s
Build (nightly)	12s
Build with minimal features (api-10, hilog)	11s
Build with API feature levels (api-12)	12s
Build with minimal features (api-10, napi)	13s
Build with minimal features (api-10, native_buffer)	14s
Build with minimal features (api-10, native_window)	15s
Build with minimal features (api-10, xcomponent)	14s
	2m 58s

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