Dotty

Improve message for nested package missing braces #37005

Job	Run time
test_windows_fast	18m 40s
test	34m 45s
MiMa	12m 20s
community_build_a	52m 7s
test_sbt	15m 38s
community_build_b	1h 22m 48s
build-sdk-package / build	10m 32s
community_build_c	1h 0m 48s
test_java8	0s
test_windows_full	0s
build-msi-package	0s
test_non_bootstrapped	0s
test-msi-package	0s
test_scala2_library_tasty	0s
build-chocolatey-package / build	20s
test-chocolatey-package	-1s
publish_release	0s
	4h 47m 57s

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