refactor: make EmitLoop error message clearer #469

Job	Run time
test magicgui	1m 27s
Check Manifest	19s
py3.7 on ubuntu-latest	1m 38s
benchmarks	2m 26s
py3.7 on macos-latest	2m 25s
typing	20s
py3.7 on windows-latest	3m 33s
py3.8 on ubuntu-latest	2m 28s
py3.8 on macos-latest	1m 48s
py3.8 on windows-latest	3m 39s
py3.9 on ubuntu-latest	2m 7s
py3.9 on macos-latest	2m 36s
py3.9 on windows-latest	2m 45s
py3.10 on ubuntu-latest	1m 29s
py3.10 on macos-latest	3m 14s
py3.10 on windows-latest	3m 9s
py3.11 on ubuntu-latest pydantic<2	2m 34s
py3.11 on macos-latest	2m 16s
py3.11 on windows-latest pydantic<2	2m 24s
py3.10 on ubuntu-latest PyQt5	1m 42s
py3.10 on macos-latest PyQt6	2m 41s
py3.10 on ubuntu-latest PySide2	2m 40s
py3.10 on macos-latest PySide6	2m 14s
py3.12-dev on ubuntu-latest	2m 28s
	54m 22s

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