feat: implement StdErr for poem::Error

[cpubot]

Implements std::error::Error for poem::Error.

This is useful for error reporting cases where, for example, it's advantageous to be able to traverse the stack of error sources for more detailed error reporting. An example of a such a case is the sentry crate's event_from_error function. Without this implementation, user's must explicitly capture every error instance in their application and forward to reporting services before wrapping them in a poem::Error, which precludes implementing a global error handler in a poem middleware (which only have access to poem::Error), for example.