llvm: Bad error message on `define` instead of `defun`

[langston-barrett]

I accidentially wrote define instead of defun and got this weird error message:

(define @f () Unit
  (return ()))

cabal run exe:grease -- test.llvm.cbl

User error: TrivialErr test.llvm.cbl:1:1

[langston-barrett]

This is mostly an upstream crucible-syntax error, but we should do better on the GREASE side indicating that it is a problem parsing the S-expression file. As you can tell from the changed labels and issue title, that was not at all obvious to me on first read.