Improve errors when DB2 database cannot be opened

When we cannot open a DB2 database, set a useful error message.
Change the signature of open_db() to to allow it to return an error
code with a message set.

(cherry picked from commit cc4f73992ec54b568c20a7f34e75f8c831dea2c9)

https://github.com/krb5/krb5/commit/8c2a7baf4f422bc0cc6dcb5507207c4191fd7f1e
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 8c2a7baf4f422bc0cc6dcb5507207c4191fd7f1e
Branch: krb5-1.14
 src/plugins/kdb/db2/kdb_db2.c |   50 +++++++++++++++++++++-------------------
 1 files changed, 26 insertions(+), 24 deletions(-)