Renumerate IPROTO error codes

This patch changes iproto codes enumeration scheme:

* 0 - OK (indicates success result of DML or ADMIN commands)
* [    1, 1<<14) - DML & ADMIN commands
* [1<<14, 1<<15) - recoverable errors (e.g. memory issue)
* [1<<15, 1<<16) - unrecoverable errors (e.g. duplicate key)
Merge remote-tracking branch 'origin/gh-344-iproto-error-codes'