This does a few things (apologies for the big PR!)

• Improves error messages for st.cache by including the cache stack and the hashing reason. In the process, I moved the place where we product the message strings inside the Exception classes, since they're not used anywhere else.
• Makes hashing errors tell the user whether the hashing error occurred when hashing the arguments, the body, or the output. See the HashReason enum.
• Makes UserHashError show up to the user not as UserHashError but as FooError, for whatever FooError was actually thrown under the hood. This way the error is easier to Google.
• Makes st.cache warnings use Exceptions in order to produce a nice stack trace, and then makes them use st.exception() protos so their output looks more similar to the exception output. But since we want warnings to show up in yellow, this also adds an is_warning field to exception protos that lets us tweak colors on the frontend.

And some smaller things:

• Makes st.exception() always read tracebacks from their input Exception, rather than allow people to pass their own exceptions (since that last code path was not used anywhere in the code).
• Does some light refactoring: makes CodeHasher a tiny bit less complex by making it not hold a hasher object internally, moves hashing-related Exception classes from one file to another, renames a couple of variables, removes some Python 2 code, etc.
• Make _HashStacks use weak references to threads rather than thread IDs, so avoid a memory leak when threads end.
• Improves the CSS for code blocks and inline code inside exceptions.