Stack details are not populated to lastAsserRawErrorString

[tyoshino]

ba07421 added support for processing AssertionError of https://www.npmjs.com/package/assert

The body of the stack item is not appended to lastAsserRawErrorString in Parser.prototype._handleError. Due to that, it gets truncated in tap-spec's output.

    ✖ AssertionError: false == true
    --------------------------------
      operator: error
      expected: |-
        undefined
      actual: |-
        { actual: false, expected: true, generatedMessage: true, message: 'false == true', name: 'AssertionError', operator: '==' }
      at: Test.exports.Test.run (/tyoshino/whatwgstreams/reference-implementation/node_modules/tape-catch/index.js:29:10)
      stack: |-