Report a useful error when someone passes an incorrect python class name.

(cherry picked from commit 9093f3c)
(cherry picked from commit 408fc46)

Author: jimingham

lldb/source/Commands/CommandObjectCommands.cpp

@@ -1599,7 +1599,8 @@ class CommandObjectCommandsScriptAdd : public CommandObjectParsed,
       auto cmd_obj_sp = interpreter->CreateScriptCommandObject(
           m_options.m_class_name.c_str());
       if (!cmd_obj_sp) {
-        result.AppendError("cannot create helper object");
+        result.AppendErrorWithFormatv("cannot create helper object for: "
+                                      "'{0}'", m_options.m_class_name);
         return false;
       }
 

lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp

@@ -1976,8 +1976,11 @@ ScriptInterpreterPythonImpl::CreateScriptCommandObject(const char *class_name) {
   PythonObject ret_val = LLDBSwigPythonCreateCommandObject(
       class_name, m_dictionary_name.c_str(), debugger_sp);
 
-  return StructuredData::GenericSP(
-      new StructuredPythonObject(std::move(ret_val)));
+  if (ret_val.IsValid())
+    return StructuredData::GenericSP(
+        new StructuredPythonObject(std::move(ret_val)));
+  else
+    return {};
 }
 
 bool ScriptInterpreterPythonImpl::GenerateTypeScriptFunction(

lldb/test/API/commands/command/script/TestCommandScript.py

@@ -164,6 +164,10 @@ def cleanup():
         # This should not crash.
         self.runCmd('bug11569', check=False)
 
+        # Make sure that a reference to a non-existent class raises an error:
+        bad_class_name = "LLDBNoSuchModule.LLDBNoSuchClass"
+        self.expect("command script add wont-work --class {0}".format(bad_class_name), error=True, substrs = [bad_class_name])
+
     def test_persistence(self):
         """
         Ensure that function arguments meaningfully persist (and do not crash!)