make format errors internal errors

include/cubescript/cubescript.hh

@@ -569,7 +569,7 @@ private:
     bool p_gap;
 };
 
-struct cs_error {
+struct LIBCUBESCRIPT_EXPORT cs_error {
     friend struct cs_state;
 
     cs_error() = delete;

src/cs_vm.cc

@@ -131,8 +131,7 @@ std::string_view cs_error::save_msg(
             );
         }
         if (sz <= 0) {
-            strncpy(cs.p_errbuf, "format error", sizeof(cs.p_errbuf));
-            sz = strlen(cs.p_errbuf);
+            throw cs_internal_error{"format error"};
         }
         return std::string_view{cs.p_errbuf, std::size_t(sz)};
     }

src/lib_str.cc

@@ -150,7 +150,7 @@ void cs_init_lib_string(cs_state &cs) {
             return;
         }
         /* should pretty much be unreachable */
-        throw cs_error{ccs, "format error"};
+        throw cs_internal_error{"format error"};
     });
 
     cs.new_command("substr", "siiN", [](auto &, auto args, auto &res) {