Remove ReadOnlyMode error struct

[Turbo87]

First converting a diesel::result::Error to a BoxedAppError only to then rely on downcasting to check the content of the original error seems a bit over-complicated. This PR replaces the check with a much simpler is_read_only_error() fn and then gets rid of the ReadOnlyMode struct in favor of another custom() error helper call.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.20%. Comparing base (7788b02) to head (f8ee4b6).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/controllers/user/session.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9968   +/-   ##
=======================================
  Coverage   89.20%   89.20%           
=======================================
  Files         292      293    +1     
  Lines       30380    30374    -6     
=======================================
- Hits        27100    27095    -5     
+ Misses       3280     3279    -1     

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