Fix wrong path separator in error message when saved model does not exist

[spopovru]

Error message generation changed for keras.models.load_model (fixed bug #44739).
Path separator recived from os.path.sep, and now for Windows error message looking like
OSError: SavedModel file does not exist at: C:\IncorrectPathToSavedModel\{saved_model.pbtxt|saved_model.pb}
for Linux -
OSError: SavedModel file does not exist at: ~/IncorrectPathToSavedModel/{saved_model.pbtxt|saved_model.pb}

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[fchollet]

Looks good, please add a quick unit test.

[bhack]

Looks good, please add a quick unit test.

Yes I think that it is better to have a quick unit test if he have enough time but also it is really trivial and it will be quite hard to create a regression there after this change.

[spopovru]

Looks good, please add a quick unit test.

Thank you!
I added a test for this bug, hopefully now all right.

[bhack]

/cc @rthadur Let's check if #43754 (comment) is still an issue if @fchollet will approve this without any extra commit request.

[spopovru]

@fchollet can you approve this pull request, please?
I was resolve issue and add simple unit test.
It's not seriously bug, off course, but this fix make project better, imho.

[fchollet]

LGTM