Always print hex for keys with corrupt results in stress tests

[ajkr]

Currently some failures look like this:

***Verification failed for column family 0 key 87500643: Value not found: NotFound:***

However this is difficult to debug since the translation from integer used to generate the key into the actual key bytes is complex and configuration-dependent. That makes it difficult to do manual investigation like looking for the key in the SST files. It'd be good to accompany the key integer with the actual key bytes (in hex format) in all error messages printed by the stress tests.