debugger: fails silently if the debug command has errors

[obycode]

For example, with the debugger-demo repo, if you try to debug the expression (contract-call? .foo hello .bat), you should see an error somewhere saying "error: use of unresolved contract 'ST1PQHQKV0RJXZFY1DGX8MNSNYVE3VGZJSRTPGZGM.bat'" but instead, it just silently dies.