[lldb] Replace asserts on .Success() with assertSuccess()

Replace forms of `assertTrue(err.Success())` with `assertSuccess(err)` (added in D82759).

* `assertSuccess` prints out the error's message
* `assertSuccess` expresses explicit higher level semantics, both to the reader and for test failure output
* `assertSuccess` seems not to be well known, using it where possible will help spread knowledge
* `assertSuccess` statements are more succinct

Differential Revision: https://reviews.llvm.org/D119616

lldb/test/API/functionalities/scripted_process/TestScriptedProcess.py

@@ -70,7 +70,7 @@ class ScriptedProcesTestCase(TestBase):
 
         process = target.Launch(launch_info, error)
 
-        self.assertTrue(error.Success(), error.GetCString())
+        self.assertSuccess(error)
         self.assertTrue(process, PROCESS_IS_VALID)
         self.assertEqual(process.GetProcessID(), 666)
         self.assertEqual(process.GetNumThreads(), 0)