Improve decryption failure message (#24573) (#24575)

Improve decryption failure message (#24573) (#24575)

Backport #24573

Help some users like #16832 #1851

There are many users reporting similar problem: if the SECRET_KEY
mismatches, some operations (like 2FA login) only reports unclear 500
error and unclear "base64 decode error" log （some maintainers ever spent
a lot of time on debugging such problem)

The SECRET_KEY was not well-designed and it is also a kind of technical
debt. Since it couldn't be fixed easily, it's good to add clearer error
messages, then at least users could know what the real problem is.