Fix stack display when using plumber

When an exception is thrown and caught by the global error handler and plumber is enabled on tasks, the stack trace and other debugging info is lost:

```
[01:40:17] Plumber found unhandled error:
 TypeError in plugin 'gulp-mocha'
Message:
    Cannot call method 'replace' of undefined
Details:
    domain: [object Object]
    domainThrown: true
    1) should load js+css on initial route
```

By passing the stack and showStack flag, the most important trace, the original throw, is properly logged.

```
[01:46:54] Plumber found unhandled error:
 TypeError in plugin 'gulp-mocha'
Message:
    Cannot call method 'replace' of undefined
Details:
    domain: [object Object]
    domainThrown: true
Stack:
TypeError: Cannot call method 'replace' of undefined
    at Tapable.replacePathVariables (/Users/kpdecker/dev/public/webpack/lib/TemplatedPathPlugin.js:66:4)
    at Tapable.applyPlugins [as applyPluginsWaterfall] (/Users/kpdecker/dev/public/webpack/node_modules/tapable/lib/Tapable.js:37:47)
    at Tapable.<anonymous> (/Users/kpdecker/dev/walmart/zeus/pack/lib/templates/main-template.js:9:4044)
    at /Users/kpdecker/dev/walmart/zeus/pack/node_modules/lodash/dist/lodash.js:901:23
    at Function.forEach (/Users/kpdecker/dev/walmart/zeus/pack/node_modules/lodash/dist/lodash.js:3297:15)
    at Tapable.<anonymous> (/Users/kpdecker/dev/walmart/zeus/pack/lib/templates/main-template.js:9:3907)
    at Tapable.applyPlugins [as applyPluginsWaterfall] (/Users/kpdecker/dev/public/webpack/node_modules/tapable/lib/Tapable.js:37:47)
    at Tapable.MainTemplate.render (/Users/kpdecker/dev/public/webpack/lib/MainTemplate.js:103:16)
    at Tapable.createChunkAssets (/Users/kpdecker/dev/public/webpack/lib/Compilation.js:751:32)
    at Tapable.<anonymous> (/Users/kpdecker/dev/public/webpack/lib/Compilation.js:509:8)
    1) should load js+css on initial route
```