Python 3: fix a bytes/str runtime issue

On python3, in grpc._links.service._Kernel._on_service_acceptance_event,
there is a runtime TypeError:

```
_on_service_acceptance_event
    group, method = service_acceptance.method.split('/')[1:3]
TypeError: 'str' does not support the buffer interface
```

It is fixed by using a bytes literal (`b'/'`) instead of a string literal.

This exposed another issue in grpc.beta._server where an exception was being
raised with a bytes literal for a message (a string should be used instead.)
diff --git a/src/python/grpcio/grpc/_links/service.py b/src/python/grpcio/grpc/_links/service.py
index 01edee6..e0f26a5 100644
--- a/src/python/grpcio/grpc/_links/service.py
+++ b/src/python/grpcio/grpc/_links/service.py
@@ -1,4 +1,4 @@
-# Copyright 2015, Google Inc.
+# Copyright 2015-2016, Google Inc.
 # All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
@@ -177,7 +177,7 @@
     call = service_acceptance.call
     call.accept(self._completion_queue, call)
     try:
-      group, method = service_acceptance.method.split('/')[1:3]
+      group, method = service_acceptance.method.split(b'/')[1:3]
     except ValueError:
       logging.info('Illegal path "%s"!', service_acceptance.method)
       return
diff --git a/src/python/grpcio/grpc/beta/_server.py b/src/python/grpcio/grpc/beta/_server.py
index 2b520cc..12d16e6 100644
--- a/src/python/grpcio/grpc/beta/_server.py
+++ b/src/python/grpcio/grpc/beta/_server.py
@@ -1,4 +1,4 @@
-# Copyright 2015, Google Inc.
+# Copyright 2015-2016, Google Inc.
 # All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
@@ -62,7 +62,7 @@
       if e.code is None and e.details is None:
         raise base.NoSuchMethodError(
             interfaces.StatusCode.UNIMPLEMENTED,
-            b'Method "%s" of service "%s" not implemented!' % (method, group))
+            'Method "%s" of service "%s" not implemented!' % (method, group))
       else:
         raise