raise error if signature is missing (#1406)

Signed-off-by: Guenther Schmuelling <[email protected]>

Author: guschmue

tf2onnx/convert.py

@@ -304,7 +304,10 @@ def from_keras(model, input_signature=None, opset=None, custom_ops=None, custom_
         An ONNX model_proto and an external_tensor_storage dict.
     """
     if LooseVersion(tf.__version__) < "2.0":
-        raise NotImplementedError("from_keras requires tf-1.15 or newer")
+        raise NotImplementedError("from_keras requires tf-2.0 or newer")
+
+    if not input_signature:
+        raise ValueError("from_keras requires input_signature")
 
     from tensorflow.python.keras.saving import saving_utils as _saving_utils # pylint: disable=import-outside-toplevel
 
@@ -364,7 +367,10 @@ def from_function(function, input_signature=None, opset=None, custom_ops=None, c
         An ONNX model_proto and an external_tensor_storage dict.
     """
     if LooseVersion(tf.__version__) < "2.0":
-        raise NotImplementedError("from_keras requires tf-1.15 or newer")
+        raise NotImplementedError("from_function requires tf-2.0 or newer")
+
+    if not input_signature:
+        raise ValueError("from_function requires input_signature")
 
     concrete_func = function.get_concrete_function(*input_signature)