Raise JSON::GeneratorError instead of Encoding::UndefinedConversionError #599

Job	Run time
Ruby memcheck	49s
ruby-versions / Generate Ruby versions	2s
ubuntu-22.04 2.7	1m 10s
ubuntu-22.04 3.0	30s
ubuntu-22.04 3.1	36s
ubuntu-22.04 3.2	28s
ubuntu-22.04 3.3	31s
ubuntu-22.04 head	1m 18s
ubuntu-22.04 jruby	1m 13s
ubuntu-22.04 jruby-head	1m 10s
macos-13 2.7	3m 6s
macos-13 3.0	37s
macos-13 3.1	38s
macos-13 3.2	43s
macos-13 3.3	37s
macos-13 head	38s
macos-13 jruby	1m 43s
macos-13 jruby-head	2m 13s
macos-14 2.7	1m 5s
macos-14 3.0	28s
macos-14 3.1	32s
macos-14 3.2	25s
macos-14 3.3	23s
macos-14 head	22s
macos-14 jruby	41s
macos-14 jruby-head	1m 14s
windows-latest 2.7	4m 44s
windows-latest 3.0	2m 16s
windows-latest 3.1	2m 37s
windows-latest 3.2	2m 2s
windows-latest 3.3	4m 3s
windows-latest head	1m 38s
windows-latest mswin	1m 44s
ubuntu-latest jruby-9.4	1m 12s
macos-latest truffleruby-head	47s
ubuntu-latest truffleruby-head	2m 2s
	46m 17s

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