Failures to add suppressions are ignored in `importCppcheckGuiProject`

[ludviggunne]

lib/importproject.cpp:

    supprs.nomsg.addSuppressions(std::move(suppressions)); // TODO: check result

This leads to suppressions in project files being silently ignored if an earlier one in the list is already supplied on the command line:

<?xml version="1.0" encoding="UTF-8"?>
<project version="1">
    <root name="."/>
    <suppressions>
        <suppression>uninitvar</suppression>
        <suppression>unreadVariable</suppression>
    </suppressions>
    <paths>
        <dir name="."/>
    </paths>
</project>

$ cppcheck --suppress=uninitvar --project=project.cppcheck

Here, unreadVariable is ignored.

[ludviggunne]

Related: #14936

[ludviggunne]

Fixed by ​https://github.com/cppcheck-opensource/cppcheck/pull/8757.