- fix a bug introduced with last commit ALTER EVENT a RENAME TO b; failed
- misc


sql/event.cc:
  - rename evex_db_find_routine_aux() to evex_db_find_event_aux() (better name)
  - change parameter order of db_update_event()
  - last bits to handle errors as close as possible to the place they occur
  - fix a bug introduced with last commit: first check for overwriting and event and then
    check whether the original one exists
sql/event.h:
  add a new error code returned by event_timed::compile() in case of 
  error
sql/event_priv.h:
  rename
sql/event_timed.cc:
  - function rename
  - add a bit of doc
sql/share/errmsg.txt:
  - extend an error message so it's more informative
  - add a new error message
sql/sql_parse.cc:
  refactor the cases for CREATE/ALTER/DROP event so use as much common
  code as possible. last bits of error checking unneeded in sql_parse.cc
  thrwon out.