db: return a value of grn_rc type instead of void #16

Job	Run time
Docker (alpine-3.20)	6m 57s
Linux (gcc, g++)	43m 29s
Linux (gcc, g++, OFF)	10m 10s
macOS	28m 54s
Linux (clang, clang++)	11m 13s
Windows MinGW (MINGW64, mingw-w64-x86_64)	18m 7s
Windows MinGW (UCRT64, mingw-w64-ucrt-x86_64)	18m 50s
Windows MSVC (windows-2022, 143, 2022)	32m 47s
Windows MSVC (windows-2019, 142, 2019)	31m 38s
	3h 22m 5s

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