How to show full stack traces?

### Expected behaviour

I'd like to see full stack traces

### Actual behaviour

I've tried Mocha, Jasmine, Chrome headless, electron, and various preprocessors, but in every case I see errors of this format:

```
Electron 2.0.0 (Node 8.9.3) ERROR
{
"message": "Uncaught TypeError: Illegal constructor\nat /home/trusktr/Downloads/src/trusktr+lowclass/src/index.js:1:11\n\nundefined",
"str": "Uncaught TypeError: Illegal constructor\nat /home/trusktr/Downloads/src/trusktr+lowclass/src/index.js:1:11\n\nundefined"
}
```

How do I enable full stack traces?

### Environment Details

- Karma version (output of `karma --version`): 2.0.2
- Relevant part of your `karma.config.js` file: Doesn't matter, the error format seems to be the same after trying different preprocessors, launchers, browsers, and frameworks.

### Steps to reproduce the behaviour

1. Run any test

---

My config looks like this (at the moment):

```js
const CWD = process.cwd()
const glob = require('globby')
const fs = require('fs')
const mkdirp = require('mkdirp')

let testFiles = null

glob([
CWD+'/src/**/*.test.js',
CWD+'/tests/**/*.js',
]).then(paths => testFiles = paths)

// wait for glob call to finish
while (!testFiles) {
require('deasync').sleep(100)
}

testFiles.forEach(file => {
const relativeFile = file.replace(CWD, '')
const relativePath = dirname(relativeFile)

mkdirp( CWD + '/.test-build' + relativePath )

fs.writeFileSync( CWD + '/.test-build' + relativeFile, `
require('@babel/register')({
presets: [ ['@babel/preset-env', { targets: { node: 9 } }] ],
sourceMap: 'inline',
})

require( '${ file }' )
` )
})

function dirname(fullname) {
const parts = fullname.split('/')
parts.pop()
return parts.join('/')
}

module.exports = function(config) {

config.set({

frameworks: ['jasmine', 'stacktrace'],
reporters: ['spec'],
port: 9876, // karma web server port
colors: true,
logLevel: config.LOG_INFO,
autoWatch: false,
concurrency: Infinity,
basePath: CWD,

browsers: ['Electron'],
files: [
'.test-build/**/*.js',
],
preprocessors: {
'.test-build/**/*.js': ['electron'],
},
client: {
// otherwise "require is not defined"
useIframe: false,
loadScriptsViaRequire: true,
},

})

}
```

Start with the reported karma.config.js settings, especially the stacktrace framework, Electron browser, electron preprocessor, and source maps in the generated .test-build files. Reproduce the issue with the listed Karma 2.0.2 environment and compare the resulting error output with the expected full stack trace.

Written by the indexing model from the issue text.