Exit with a special code for tool/browser failure

### Expected behavior

If karma tests fail due to browser issue karma single run should exit with an exit code to differentiate between a 'tool failure' vs. a 'test failure'.
Would like karma to 'exit(100)' to indicate this type of failure or the error code must be configurable for these situations.
### Actual behavior

Karma always exits with a '1' status whenever anythinng goes wrong, test failure or browser failure
### Enviroment Details
- Karma version (output of `karma --version`):
- Relevant part of your `karma.config.js` file

Start by tracing Karma's single-run exit handling and how browser failures are distinguished from test failures. Determine where the current status 1 is produced and define how a separate or configurable tool-failure code should behave; done means single runs reliably return distinguishable statuses without changing test-failure behavior.

Written by the indexing model from the issue text.