paths: add a better error message for posix expansion

paths_posix.cpp

@@ -25,7 +25,7 @@ cruft::paths::expand (std::filesystem::path const &val)
     std::unique_ptr<wordexp_t, decltype(&wordfree)> cleanup {&words, &wordfree};
 
     if (words.we_wordc != 1)
-        throw std::runtime_error ("glob matches too many files");
+        throw std::runtime_error ("path expansion is not singular");
 
     CHECK_EQ (words.we_offs, 0u);
     return words.we_wordv[0];