exc: Add {Backend,Frontend,Mixer}Error exceptions

(cherry picked from commit bf8307f329)

mopidy/exceptions.py

@@ -16,9 +16,21 @@ class MopidyException(Exception):
         self._message = message
 
 
+class BackendError(MopidyException):
+    pass
+
+
 class ExtensionError(MopidyException):
     pass
 
 
+class FrontendError(MopidyException):
+    pass
+
+
+class MixerError(MopidyException):
+    pass
+
+
 class ScannerError(MopidyException):
     pass

tests/test_exceptions.py

@@ -12,10 +12,22 @@ class ExceptionsTest(unittest.TestCase):
         self.assertEqual(exc.message, 'foo')
         self.assertEqual(str(exc), 'foo')
 
+    def test_backend_error_is_a_mopidy_exception(self):
+        self.assert_(issubclass(
+            exceptions.BackendError, exceptions.MopidyException))
+
     def test_extension_error_is_a_mopidy_exception(self):
         self.assert_(issubclass(
             exceptions.ExtensionError, exceptions.MopidyException))
 
+    def test_frontend_error_is_a_mopidy_exception(self):
+        self.assert_(issubclass(
+            exceptions.FrontendError, exceptions.MopidyException))
+
+    def test_mixer_error_is_a_mopidy_exception(self):
+        self.assert_(issubclass(
+            exceptions.MixerError, exceptions.MopidyException))
+
     def test_scanner_error_is_a_mopidy_exception(self):
         self.assert_(issubclass(
             exceptions.ScannerError, exceptions.MopidyException))