Change error message when trying connection with more than 30 characters username

[AthAshino]

Hello, I discovered today that we can't use username longer than 30 characters. I can't find it in the documentation. The only reason I found out this problem is by reading the code, which cost me many hours. The message that appears when using a too long username is "Invalid username" ("Invalid password" for password), which is not revelant.

I think it should be something like "Too long username (more than 30 characters".

Also, why is there this limitation? And why can't we change it in the configuration? Thank you