addsimp with non-equation theorem causes REPL to exit

[brianhuffman]

Function ruleOfProp calls error due to a problem with a user-provided rewrite rule. This should be a user-level error, not a panic. For an example, see #511, or run the following commands at the saw repl:

sawscript> thm <- prove_print z3 {{ True }}
[15:34:02.187] Valid
sawscript> addsimp thm empty_ss
[15:34:06.810] Rewrite Rules
=============
saw: ruleOfProp: Predicate not an equation: Prelude.True
CallStack (from HasCallStack):
  error, called at src/Verifier/SAW/Rewriter.hs:306:16 in saw-core-0.1-9trtP7WRb9qGDvQurwxUK4:Verifier.SAW.Rewriter

[brianhuffman]

Duplicate of #319.