Do not Rethrow DoNotRetryIOException as UnknownScannerException

HBase commit https://github.com/apache/hbase/commit/94ade6a514e9935a8c283befb31f29cd8d3a2045 broke co-processors (such as Phoenix) throwing DoNotRetryIOExceptions when scanning a table. This change rethrows them as UnknownScannerExceptions which the HBase client will retry on. This is unintended behavior since co-processors should be able to throw DoNotRetryIOExceptions back to the client. This came up through a phoenix IT when trying to upgrade to HBase 1.3.0 https://github.com/apache/phoenix/pull/230