Browser: fix 'allowUncaught' handling

[juergba]

Description

• mocha/browser-entry.js: Node's process.on('uncaughtException', fn) handler is moved to global.onerror(fn) in Browser.

Runner.prototype.uncaught = function(err) {
  if (err instanceof Pending) {
    return;
  }
  // browser does not exit when throwing in global.onerror()
  if (this.allowUncaught && !process.browser) {
    throw err;
  }
[...]

• the result is distinct when an Error is thrown in the handler and allowUncaught: true
  • Node: the process exits and no tests or async tasks are executed anymore.
  • Browser: the JS script aborts the handler only, but the script keeps running. There seems no way to kill the entire script with its remaining tests and pending async tasks.
    ==> test results are incorrect, since the entire recovery logic of the handler is skipped.

Description of the Change

• Node: behavior remains unchanged, the process is aborted upon uncaughtException when allowUncaught is set.
• Browser: we don't throw anymore inside the uncaughtException handler. The script keeps running and the test results are correct. So allowUncaught has no effect on the test run, but uncaughtException's are printed in the console.

[coveralls]

Coverage remained the same at 92.857% when pulling 0bb3e5f on juergba/browser into 2ff1cb2 on master.