Add margins to yarn exception excerpts

The yarn tests check the log files for exceptions to detect test failures. If detected a test will fail and an excerpt from the logs will be printed.

The excerpt content is currently the stack of the detected exception. This only works correctly if the stacktrace follows a specific formatting style; for example if an exception message contains an empty line the output will be cut off.

I propose including the 10 before/after the found exception to make this a bit more reliable. As a side-effect we also get a little contextual information.