Adding docs and tests for `try`.
Defining exception types.
Teaching the evaluator new-style exceptions.
Teaching the top-level about new-style exceptions.
Changing `/` to throw new-style exceptions.
Adding a more helpful __repr__ on TrifleExceptionInstance for debugging.
Expanding `try` tests.
Include error types in the global scope by default.
Adding a skeleton `try` implementation.

This evaluates the body and the expected exception type, but does not
actually catch exceptions yet.
Adding .repr to error types.
Fixing inverted logic in Stack.is_empty.
Expanding `try` tests.
Catch errors when we encounter them! Another major milestone!
Fixing failing test.
Fixing build.
Using more appropriate string formatting.
Renaming errors so their names don't always end with -error.
Implementing a base exception, and allowing it to be caught.
To avoid confusion, ensure we always call .eval with strings.
Defining a type error and an arity error for our new exception system.