refactor(webserver): always use schema::Result for graphql api responses

[wsxiaoys]

cc @liangfung

[codecov]

Codecov Report

Attention: Patch coverage is 12.50000% with 21 lines in your changes are missing coverage. Please review.

Project coverage is 56.05%. Comparing base (25f47a7) to head (7215c2a).
Report is 8 commits behind head on main.

Files	Patch %	Lines
ee/tabby-webserver/src/juniper/relay/mod.rs	21.42%	11 Missing ⚠️
ee/tabby-webserver/src/schema/mod.rs	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1892      +/-   ##
==========================================
+ Coverage   55.42%   56.05%   +0.63%     
==========================================
  Files         125      125              
  Lines       11013    11099      +86     
==========================================
+ Hits         6104     6222     +118     
+ Misses       4909     4877      -32     

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