Streamlit: When "streamlit run" file doesn't exit and has no extension, error printout is weird

[tvst]

1. Create a Python file and call it example (without .py)
2. streamlit run example

Here's what you get:

This error message is not a valid sentence: "Streamlit requires raw Python (.py) files, not ."

What's happening is that the code is trying to write the file extension in the error message, but in this case the file has no extension.

We should instead say something like "Streamlit requires raw Python (.py) files, and the provided file has no extension."

[abhinand5]

I can fix this problem. If approved, I will make a pull request.

[randyzwitch]

Thanks for volunteering @abhinand5, please feel free to take a shot at it and submit a PR.

[abhinand5]

Thanks @randyzwitch, I have submitted the PR