InnoDB used to use a mixture of int, ulint, and enum db_err for error
codes. This patch changes it to use dberr_t (an enum) everywhere, so
that errors are caught more easily. For example, one could forget to
convert an InnoDB error code to an MySQL error code, or do the
conversion twice. Also, gdb will nicely display the symbolic names of
error codes when the enum type is being used.

Also, the definition of trx->mysql_thd as void* instead of THD* causes
unnecessary type casts.

While we are at it, use ut_strerr() for reporting InnoDB internal
errors to the user. In this way, the enum values can be changed at
will.

rb:1026 approved by Jimmy Yang