Calling std::locale("fr_FR.UTF-8") crashes on Linux and Mac. Or
rather, it throws an exception when it doesn't know the locale and we
didn't handle the exception. I now catch the exception and output an
error rather than crash.

Note, this happened because of change 703f3bcf7069, which made us
actually pass a real locale string instead of just "C". So, we were
never actually supporting a locale for LLStringUtil::formatNumber().
There is therefore an open task of making formatNumber() actually
respect the locale. I'll report a separate JIRA to capture that task.