[sumac] fix: Update error messages when adding user to library [FC-0062] (#1543)

[ChrisChV]

Backport of #1543

Cherry-pick of d2d7532

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Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@                     Coverage Diff                      @@
##             open-release/sumac.master    #1550   +/-   ##
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  Coverage                             ?   93.23%           
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  Files                                ?     1050           
  Lines                                ?    20418           
  Branches                             ?     4362           
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  Hits                                 ?    19036           
  Misses                               ?     1325           
  Partials                             ?       57           

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[bradenmacdonald]

@farhaanbukhsh FYI - bugfix backport.

[bradenmacdonald]

LGTM and is working well when I tested it on my Sumac devstack.