Warn or error for Invalid kernel names

[digital-carver]

I created a kernel with parantheses (...) in its name, and it seemed to work fine until I tried to start Jupyter with it.

UserWarning: Invalid kernelspec directory name (Kernel names can only contain ASCII letters and numbers and these separators: - . _ (hyphen, period, and underscore).)

Discourse thread (from someone else) with the same issue

Since Jupyter helpfully lists the characters allowed in the name, it would be better to alert the user with a warning, or better an error, instead of silently doing the wrong thing. The last message in #614 hints at the fact that you can use specname to circumvent this, but the user first has to know that this is an issue, and that this can be a solution. An error message seems a good way to convey this information, and to allow the user to quickly make the correction, rather than create the kernel and find out much later that it has this problem.

[stevengj]

Yes, seems like a good idea.