Throw a LadesaeulenValidationError with the message Invalid <name>: <reason>
when problem(value) finds a reason; otherwise return value unchanged. Call it
before any request, so a rejected input sends nothing. Async methods call it
inside their body, so the rejection arrives as a rejected promise rather than a
synchronous throw; constructors throw.
Throw a LadesaeulenValidationError with the message
Invalid <name>: <reason>whenproblem(value)finds a reason; otherwise returnvalueunchanged. Call it before any request, so a rejected input sends nothing. Async methods call it inside their body, so the rejection arrives as a rejected promise rather than a synchronous throw; constructors throw.