change handle_error to handle_user_error

[laralove143]

while there isn't many ways to hande user errors, there's many ways to handle internal errors, you may want to use a tracing crate, log your own errors for metrics etc

• deprecates handle_error since the only addition would be to simply log the errors with Bot::log
• removes printing and file-logging functionality, since those aren't discord-related

closing #12 in favor of this