PROBLEM:
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1) When we start up 8.0 server on a 5.7 data directory
containing orphan events (i.e. events whose database no
longer exists), the server upgrade crashes due to an invalid
reference.

2) Additionally, attempting to upgrade orphan stored
routines fails with error messages that do not clearly
identify the cause of failure.

When the database is not found, db_load_routine() returns
enum_sp_return_code::SP_INTERNAL_ERROR, instead of the more
appropriate enum_sp_return_code::SP_NO_DB_ERROR.

FIX:
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1) The server upgrade now fails gracefully with an
appropriate error indicating an orphan event.

2) An error is shown when the database is not found,
indicating an orphan routine.

Change-Id: Id3ad0bfa8572b1d53c67264f8fb8a5a775c2af3f