[processing] Don't append full traceback when a QgsProcessingException is

raised by a Python algorithm

This is too noisy for these expected exceptions -- instead, only
show the traceback for other exceptions (Which are likely a result
of Python coding errors, so they are useful for debugging)

python/core/core.sip.in

@@ -107,8 +107,40 @@ done:
 %Include core_auto.sip
 
 %VirtualErrorHandler processing_exception_handler
-    QString trace = getTraceback();
+    // if an explicit QgsProcessingException was raised, we don't retrieve
-    QgsLogger::critical( trace );
+    // and append the trace. It's too noisy for these "expected" type errors.
+    // For all other exceptions, it's likely a coding error in the algorithm,
+    // so we DO retrieve the full traceback for debugging.
+    PyGILState_STATE gstate;
+    gstate = PyGILState_Ensure();
+    PyTypeObject* err = reinterpret_cast< PyTypeObject* >( PyErr_Occurred() );
+    const bool isProcessingException = err && QString( err->tp_name ) == QStringLiteral( "QgsProcessingException" );
+
+    QString what;
+    if ( isProcessingException )
+    {
+      PyObject *type, *value, *traceback;
+      PyErr_Fetch( &type, &value, &traceback );
+      // check whether the object is already a unicode string
+      if ( PyUnicode_Check( value) )
+      {
+        what = QString::fromUtf8( PyUnicode_AsUTF8( value ) );
+      }
+      else
+      {
+        PyObject* str = PyObject_Str( value );
+        what = QString::fromUtf8( PyUnicode_AsUTF8( str ) );
+        Py_XDECREF( str );
+      }
+      PyGILState_Release( gstate );
+    }
+    else
+    {
+      PyGILState_Release( gstate );
+      QString trace = getTraceback();
+      QgsLogger::critical( trace );
+      what = trace;
+    }
     SIP_RELEASE_GIL( sipGILState );
-    throw QgsProcessingException( trace );
+    throw QgsProcessingException( what );
 %End