panic: entered unreachable code: Missing SCM: JustFail

[langston-barrett]

Found with tree-crasher on 0e01a74.

/[]*1/u.exec();

thread 'main' panicked at 'internal error: entered unreachable code: Missing SCM: JustFail', /home/langston/.cargo/registry/src/github.com-1ecc6299db9ec823/regress-0.5.0/src/classicalbacktrack.rs:307:17
stack backtrace:
   0: rust_begin_unwind
             at /rustc/2c8cc343237b8f7d5a3c3703e3a87f2eb2c54a74/library/std/src/panicking.rs:575:5
   1: core::panicking::panic_fmt
             at /rustc/2c8cc343237b8f7d5a3c3703e3a87f2eb2c54a74/library/core/src/panicking.rs:64:14
   2: regress::classicalbacktrack::MatchAttempter<Input>::try_at_pos
   3: boa_engine::builtins::regexp::RegExp::abstract_builtin_exec
   4: boa_engine::builtins::regexp::RegExp::exec
   5: boa_engine::vm::code_block::<impl boa_engine::object::jsobject::JsObject>::call_internal
   6: boa_engine::object::internal_methods::function::function_call{{reify.shim}}
   7: <boa_engine::vm::opcode::call::Call as boa_engine::vm::opcode::Operation>::execute
   8: boa_engine::vm::<impl boa_engine::context::Context>::run
   9: boa_engine::context::Context::execute
  10: boa_engine::context::Context::eval_script
  11: boa::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[jasonwilliams]

This one may just be a case of reporting the regex failure to Regress.
Obviously test it outside of Boa context first

[postmeback]

Can you give me some context on this? I would love to get started on this.