Add LDAP error message explanation. (rebased onto dev_5_0)



This is the same as gh-814 but rebased onto dev_5_0.

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This is a change that should make clear that besides error messages thrown by OMERO, there is a set of Spring- and Java-specific messages that have to be looked up in external sources (i.e. Spring API docs and Java API docs). See https://www.openmicroscopy.org/community/viewtopic.php?f=4&t=7426#p14069.