Debug karma.error is not a function

### Expected behavior

When karma is run in debug and a karma adapter calls `__karma__.error`, the error message should be printed on the console.
### Actual behavior

When karma is run in debug, debug.html defines the `window.__karma__` object but does not include an error function. Then, when a karma adapter calls `__karma__.error`, the error message printed is `karma.error is not a function` which occludes the real error.

There is at least one [occurrence](https://github.com/rolaveric/karma-systemjs/issues/40) reported in the wild.
### Enviroment Details
- Karma version (output of `karma --version`): 0.13.22
- Relevant part of your `karma.config.js` file

> frameworks: ['jspm', ...]

or other adapter which calls karma.error.
### Steps to reproduce the behaviour
1. Mock an adapter which alway calls window.**karma**.error with a message.
2. Start karma on any test suite in debug.
3. The intended error message is not printed on the console.

Start with debug.html, where the window.__karma__ object is defined, and compare its available callbacks with the adapter API. Reproduce the issue using an adapter that calls window.__karma__.error in debug mode, then verify that the adapter's intended error message appears in the console instead of a missing-function error.

Written by the indexing model from the issue text.