test-server: exit when child process failed

Improve handling of errors reported by the spawned child:
 - include more info in the error message printed to console
 - exit the process (it is kept alive by the http server)
npm test: remove dependency on global karma-cli

The npm module karma does not expose the "karma" binary via package.json
bin scripts, developers are advised to install global karma-cli.

To make the life of CI server easier, the "npm test" script is modified
to "./node_modules/karma/bin/karma" instead of "karma".