Taking care of the case when no secret file exits

[alexfilothodoros]

Instead of having an ugly error message like this:

I added an exception. Now a simpler and more direct message notifies the user that there's something wrong with the secrets.toml file.

[anoopshrma]

I think we can check for the secrets with something like this in the home page

if not st.secrets.get(Secret key name):
      use st.info to show that key is not added

and if secrets are there we can put a flag in st.session then before accessing the secrets we can check the flag

[alexfilothodoros]

That's fine by me. @jerryjliu ?