Degrade gracefully when there are some parse errors

[twada]

Instead of hard error, show syntax error when there are some parse errors caused by not-supported syntax.

example:

  assert.deepEqual({ b, ...obj }, { a, ...obj })
                        ?                       
                        ?                       
                        SyntaxError: Unexpected token (1:22)

  If you are using `babel-plugin-espower` and want to use experimental syntax in your assert(), you should set `embedAst` option to true.
  see: https://github.com/power-assert-js/babel-plugin-espower#optionsembedast