Move oauth2 error to oauth2 service package

[lunny]

As title.

[codecov-commenter]

Codecov Report

Merging #17603 (6007e3e) into main (30515f2) will increase coverage by 0.00%.
The diff coverage is 61.55%.

@@           Coverage Diff           @@
##             main   #17603   +/-   ##
=======================================
  Coverage   45.48%   45.49%           
=======================================
  Files         793      792    -1     
  Lines       88846    88857   +11     
=======================================
+ Hits        40416    40423    +7     
+ Misses      41918    41917    -1     
- Partials     6512     6517    +5     

Impacted Files	Coverage Δ
models/issue_dependency.go	63.76% <0.00%> (ø)
models/lfs_lock.go	57.14% <0.00%> (ø)
models/notification.go	65.28% <0.00%> (-0.88%)	⬇️
models/repo_collaboration.go	57.89% <0.00%> (ø)
modules/doctor/fix16961.go	38.76% <0.00%> (ø)
routers/api/v1/repo/mirror.go	0.00% <0.00%> (ø)
routers/api/v1/repo/transfer.go	60.46% <0.00%> (ø)
routers/init.go	80.80% <0.00%> (ø)
routers/web/admin/auths.go	48.22% <0.00%> (ø)
routers/web/explore/code.go	0.00% <0.00%> (ø)
... and 73 more

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