* Simplify code.
* Show backtraces for all users, not just builders.
* Show backtraces only for unexpected server errors (status 5xx), not
for normal client errors (status 4xx).
* Log expected errors at info level (reduce noise in production logs).
Fixes an "incompatible character encodings: UTF-8 and ASCII-8BIT"
exception when uploading a deleted Pixiv post.
Caused by the Pixiv API error message not having the encoding set to UTF-8.