Ticket #47919 - ldbm_back_modify SLAPI_PLUGIN_BE_PRE_MODIFY_FN does not return even if one of the preop plugins fails.

Bug Description: In ldbm_back_modify, if one of the be_preop plugins
called at SLAPI_PLUGIN_BE_PRE_MODIFY_FN fail, ldbm_back_modify does
not return there but go to the real modify operation.

Fix Description: If an error is returned from be_preop plugins in
ldbm_back_modify, it returns the error there and does not proceed
the modify operation.

https://fedorahosted.org/389/ticket/47919

Reviewed by mreynolds@redhat.com (Thank you, Mark!!)

(cherry picked from commit cb173814ec579fafbfef515738c16a986a9749cd)
(cherry picked from commit 17294432e95bdb30c6816cefa7be5035df818a43)
(cherry picked from commit 65c73b4f909903b46655aee020b2e82c0cd4cfd0)