Add usability error message for "throw expr" without parens #934

Job	Run time
ubu-22 \| clang++-15 \| c++20 \| libstdc++ \| ubuntu-22.04	5m 59s
ubu-24 \| clang++-19 \| c++23 \| libc++-18-dev \| ubuntu-24.04	8m 17s
ubu-22 \| clang++-15 \| c++20 \| libc++-15-dev \| ubuntu-22.04	6m 18s
mac-13 \| clang++ \| c++2b \| default \| macos-13	1s
ubu-24 \| clang++-19 \| c++20 \| libstdc++ \| ubuntu-24.04	7m 12s
win-22 \| cl.exe \| c++20 \| default \| windows-2022	28s
ubu-24 \| g++-14 \| c++2b \| libstdc++ \| ubuntu-24.04	6m 56s
ubu-24 \| g++-13 \| c++2b \| libstdc++ \| ubuntu-24.04	6m 5s
mac-13 \| clang++-15 \| c++2b \| default \| macos-13	1s
mac-14 \| clang++ \| c++2b \| default \| macos-14	2m 45s
win-22 \| cl.exe \| c++latest \| default \| windows-2022	29s
aggregate-results	0s
	44m 31s

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