Fix ImportError when using generator/protoc with Python 3
diff --git a/generator/nanopb_generator.py b/generator/nanopb_generator.py
index d9017d1..9b37cd5 100755
--- a/generator/nanopb_generator.py
+++ b/generator/nanopb_generator.py
@@ -58,7 +58,7 @@
          ****************************************************************************
     ''' + '\n')
     raise
-except (ValueError, SystemError):
+except (ValueError, SystemError, ImportError):
     # Probably invoked directly instead of via installed scripts.
     import proto.nanopb_pb2 as nanopb_pb2
     from proto._utils import invoke_protoc