Make the PSQLError description more useful

[trasch]

... by making it conform to CustomStringConvertible and CustomDebugStringConvertible.

Currently, print(error as! PSQLError) will print something like this:
PSQLError(backing: PostgresNIO.PSQLError.(unknown context at $100aeb99c).Backing)
which is not very helpful.

I would propose to add a description to PSQLError that uses some of the infos provided in .code, .serverInfo and other properties to convey a more helpfull message.

And I can tackle this myself, unless someone else wants to do it... 😄

[fabianfett]

Let's discuss here... And I'm happy for you to take it from there:
#356

[trasch]

This here suits me at least: https://github.com/Outdooractive/PostgresConnectionPool/blob/main/Sources/PostgresConnectionPool/Extensions/PSQLError%2BDescription.swift