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