[Telemetry] Fix NotImplementedError exceptions.

tools/telemetry/telemetry/core/backends/app_backend.py

Index: tools/telemetry/telemetry/core/backends/app_backend.py
diff --git a/tools/telemetry/telemetry/core/backends/app_backend.py b/tools/telemetry/telemetry/core/backends/app_backend.py
index 64314ca902c29b21869752f8450c8edc16913f05..b74d96041f31dcdc03257be5a2dc0aab726992b1 100644
--- a/tools/telemetry/telemetry/core/backends/app_backend.py
+++ b/tools/telemetry/telemetry/core/backends/app_backend.py
@@ -23,21 +23,21 @@ class AppBackend(object):
   def app_type(self):
     return self._app_type
 
+  @property
+  def pid(self):
+    raise NotImplementedError
+
   def Start(self):
-    NotImplementedError()
+    raise NotImplementedError
 
   def Close(self):
-    NotImplementedError()
-
-  @property
-  def pid(self):
-    NotImplementedError()
+    raise NotImplementedError
 
   def IsAppRunning(self):
-    NotImplementedError()
+    raise NotImplementedError
 
   def GetStandardOutput(self):
-    NotImplementedError()
+    raise NotImplementedError
 
   def GetStackTrace(self):
-    NotImplementedError()
+    raise NotImplementedError