Do not let exception propagate out of register restoring lambda

It is called during a std::vector destruction, which is noexcept,
leading to terminate being called.

src/commands.cc

@@ -1563,7 +1563,14 @@ void context_wrap(const ParametersParser& parser, Context& context, Func func)
     auto& register_manager = RegisterManager::instance();
     auto make_register_restorer = [&](char c) {
         return on_scope_end([&, c, save=register_manager[c].get(context) | gather<Vector<String>>()] {
+            try
+            {
                 RegisterManager::instance()[c].set(context, save);
+            }
+            catch (runtime_error& err)
+            {
+                write_to_debug_buffer(format("failed to restore register '{}': {}", c, err.what()));
+            }
         });
     };
     Vector<decltype(make_register_restorer(0))> saved_registers;

src/utils.hh

@@ -73,7 +73,7 @@ public:
     { other.m_valid = false; }
 
     [[gnu::always_inline]]
-    ~OnScopeEnd() { if (m_valid) m_func(); }
+    ~OnScopeEnd() noexcept(noexcept(std::declval<T>()())) { if (m_valid) m_func(); }
 
 private:
     bool m_valid;