fix: Gracefully handle adding users when backend is offline [WPB-1100]

[atomrc]

WPB-1100 [Web] Implement Proteus client protocol for adding users in offline backend

Description

Will fix error reporting when users are added in a conversation that contains some users from an offline backend.

see wireapp/wire-web-packages#5489

Checklist

• PR has been self reviewed by the author;
• Hard-to-understand areas of the code have been commented;
• If it is a core feature, unit tests have been added;

[codecov]

Codecov Report

Merging #15785 (882ef12) into dev (e557754) will decrease coverage by 0.01%.
Report is 6 commits behind head on dev.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev   #15785      +/-   ##
==========================================
- Coverage   44.47%   44.47%   -0.01%     
==========================================
  Files         672      672              
  Lines       22707    22714       +7     
  Branches     5168     5169       +1     
==========================================
+ Hits        10100    10103       +3     
- Misses      11318    11321       +3     
- Partials     1289     1290       +1