Tests: Improve RunCMake.CPack*.SOURCE_PACKAGE test error formatting

Format message content for `message(FATAL_ERROR)`.

Apply the change from commit c54c284de7 (Tests: Improve RunCMake.CPack*
test error formatting, 2021-04-22) to the `SOURCE_PACKAGE` script too.
Also fix a typo from commit f5d79dec53 (Tests: Cleanup CPack message when
files are expected and none are found, 2021-01-15, v3.20.0-rc1~89^2~22).
2 files changed