YarnTestBase should print prohibited string

The yarn tests check the log files for a set of prohibited strings. If found, the entire log file is logged as WARN, the offending line is logged as ERROR, and the test fails with this unhelpful message:

java.lang.AssertionError(Found a file /home/travis/build/apache/flink/flink-yarn-tests/target/flink-yarn-tests-fifo/flink-yarn-tests-fifo-logDir-nm-1_0/application_1510164935122_0002/container_1510164935122_0002_01_000001/jobmanager.log with a prohibited string: [Exception, Started SelectChannelConnector@0.0.0.0:8081])

If you don't have log access on travis you have thus no knowledge what actually went wrong.

I propose to also print smaller excerpts around the found error (like 10 lines or smth) in the Assert.fail message.