Add useful error msg in `GodotSoftBody3D::set_mesh` #30284

Job	Run time
📊 Static checks / Code style, file formatting, and docs	27s
🤖 Android / Template (target=template_release)	30m 5s
🐧 Linux / Editor w/ Mono (target=editor)	34m 57s
🍏 iOS / Template (target=template_release)	10m 42s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	43m 22s
🏁 Windows / Editor (target=editor, tests=yes)	41m 4s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	29m 39s
🏁 Windows / Template (target=template_release)	32m 6s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	28m 9s
🍎 macOS / Editor (target=editor, tests=yes)	28m 23s
🍎 macOS / Template (target=template_release)	17m 57s
🐧 Linux / Template w/ Mono (target=template_release)	24m 46s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	23m 8s
🐧 Linux / Minimal template (target=template_release, everything disabled)	13m 56s
🌐 Web / Template w/o threads (target=template_release, threads=no)	23m 6s
🪲 Godot CPP / Build and test Godot CPP	7m 2s
	6h 28m 49s

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