Improve some proc-macro attribute error messages #3014

Job	Run time
clippy	38s
godot-itest (macos)	1m 1s
doc-lints	40s
license-guard	34s
godot-itest (windows)	3m 18s
run-examples	1m 4s
godot-itest (linux-full)	2m 7s
rustfmt	21s
unit-test	2m 13s
godot-itest (linux-features-experimental)	1m 2s
godot-itest (linux-4.1)	59s
godot-itest (linux-4.2)	1m 18s
godot-itest (linux-memcheck)	2m 10s
ci-status	1s
	17m 26s

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