Improve diagnostic for attempt to use `get_name` with non-external #295

Job	Run time
windows (msvc-14.0, 14,latest, 32,64, windows-2019, -j3)	1m 30s
windows (gcc, 03,11,14,17,2a, 64, windows-2019, -j3)	7m 28s
posix (gcc-12, 03,11,14,17,2a, ubuntu-22.04, cxxflags=--coverage -fsanitize=address,leak,undefine...	6m 22s
windows (msvc-14.3, 20,latest, 64, windows-2022, -j1)	13m 49s
posix (gcc-11, 03,11,14,17,2a, ubuntu-22.04, cxxflags=--coverage -fsanitize=address,leak,undefine...	6m 0s
posix (gcc-10, 03,11,14,17,2a, ubuntu-22.04, cxxflags=--coverage -fsanitize=address,leak,undefine...	7m 10s
posix (gcc-9, 03,11,14,17,2a, ubuntu-22.04, cxxflags=--coverage -fsanitize=address,leak,undefined...	5m 39s
posix (clang-14, 03,11,14,17,20, ubuntu-22.04)	6m 30s
posix (clang-13, 03,11,14,17,20, ubuntu-22.04)	6m 16s
finish	0s
	1h 0m 44s

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