Avoid propagating msg unpack exceptions to event loop

This change addresses conditions where an exception is thrown by the library upon receipt of a malformed message from an external
source, and propagated up to the event loop with no chance of
interception by the application. This issue is only experienced by
proxy convenience APIs, as low-level APIs allow the application to
unpack the message.

Strategy:
1. For malformed signals received by proxies: ignore the signal.
2. For malformed async method responses, translate the unpack
  exception into an sdbus::Error, and pass it to the caller as expected.

include/sdbus-c++/ConvenienceApiClasses.inl

@@ -588,7 +588,20 @@ namespace sdbus {
 
             // Deserialize input arguments from the message into the tuple (if no error occurred).
             if (error == nullptr)
-                reply >> args;
+            {
+                try
+                {
+                    reply >> args;
+                }
+                catch (const sdbus::Error& e)
+                {
+                    // Catch message unpack exceptions and pass them to the callback
+                    // in the expected manner to avoid propagating them up the call
+                    // stack to the event loop.
+                    sdbus::apply(callback, &e, args);
+                    return;
+                }
+            }
 
             // Invoke callback with input arguments from the tuple.
             sdbus::apply(callback, error, args);
@@ -628,7 +641,17 @@ namespace sdbus {
             tuple_of_function_input_arg_types_t<_Function> signalArgs;
 
             // Deserialize input arguments from the signal message into the tuple
-            signal >> signalArgs;
+            try
+            {
+                signal >> signalArgs;
+            }
+            catch (const sdbus::Error& e)
+            {
+                // The convenience API callback cannot handle an incoming signal with
+                // an unexpected payload, so catch and ignore this exception to avoid
+                // propagating it up the call stack to the event loop.
+                return;
+            }
 
             // Invoke callback with input arguments from the tuple.
             sdbus::apply(callback, signalArgs);