HA_ERR was returning 0 (null string) when no error happened 
(error=0). Since HA_ERR is used in DBUG_PRINT, regardless there 
was an error or not, the server could crash in solaris debug
builds.

We fix this by:

  - deploying an assertion that ensures that the function 
    is not called when no error has happened;
  - making sure that HA_ERR is only called when an error 
    happened;
  - making HA_ERR return "No Error", instead of 0, for 
    non-debug builds if it is called when no error happened.

This will make HA_ERR return values to work with DBUG_PRINT on
solaris debug builds.