Test exception handling better/easier than testThreadInterruptDeadlock() [LUCENE-4460]

currently the fact that MockDirectoryWrapper has throttlers and such that sleep, combined with the fact this test interrupts, makes a good test for exception handling.

The problem is this is really crappy to debug: things dont reproduce, you have to use hundreds or thousands of iterations, etc etc.

I think it would be better if we made it possible for MockIndexInput to throw random exceptions like MockIndexOutput and had a single threaded test that just threw random exceptions? This way it would reproduce...

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Migrated from [LUCENE-4460](https://issues.apache.org/jira/browse/LUCENE-4460) by Robert Muir (@rmuir)

Start by reading testThreadInterruptDeadlock() and the exception-throttling behavior in MockDirectoryWrapper. Compare MockIndexInput with MockIndexOutput to understand how random exceptions are injected. Done should be a single-threaded test that exercises exception handling reproducibly without relying on interrupt timing or hundreds of iterations.

Written by the indexing model from the issue text.