Raise NotImplementedError exceptions correctly.

BUG=439346

Review URL: https://codereview.chromium.org/761863007

Cr-Commit-Position: refs/heads/master@{#307055}

tools/telemetry/telemetry/core/backends/android_app_backend.py

@@ -8,3 +8,22 @@ from telemetry.core.backends import app_backend
 class AndroidAppBackend(app_backend.AppBackend):
   def __init__(self):
     super(AndroidAppBackend, self).__init__()
+
+  @property
+  def pid(self):
+    raise NotImplementedError
+
+  def Start(self):
+    raise NotImplementedError
+
+  def Close(self):
+    raise NotImplementedError
+
+  def IsAppRunning(self):
+    raise NotImplementedError
+
+  def GetStandardOutput(self):
+    raise NotImplementedError
+
+  def GetStackTrace(self):
+    raise NotImplementedError

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