Add handler for Process.onError in test_runner.dart.
Add handler for Process.onError in test_runner.dart.

The handler prints informaion, and does nothing.  This reproduces the current behavior with no handler, and is a first step to handling this error correctly.

BUG=dart:2726
TEST=tools/test.dart (Selenium and Browser tests?)

Review URL: https://chromiumcodereview.appspot.com//10228007

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@6990 260f80e4-7a28-3924-810f-c04153c831b5

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