[bootstrap] show warning but not fail when install llvm@9 failed (#5751)
[bootstrap] show warning but not fail when install llvm@9 failed (#5751)

This commit allows bootstrap not to fail but print warning if
installing llvm@9 or clang-format-9 or clang-tidy-9 failed. This has
been the best practice for installing other pretty tools.
srickardti/openthread,abtink/openthread,chshu/openthread,srickardti/openthread,srickardti/openthread,abtink/openthread,openthread/openthread,bukepo/openthread,bukepo/openthread,chshu/openthread,chshu/openthread,abtink/openthread,abtink/openthread,jwhui/openthread,openthread/openthread,jwhui/openthread,chshu/openthread,chshu/openthread,openthread/openthread,bukepo/openthread,bukepo/openthread,srickardti/openthread,chshu/openthread,openthread/openthread,jwhui/openthread,jwhui/openthread