Replace panicing APIs with such returning errors

[sftse]

This doesn't really affect me, since I only use the APIs that return errors, but it seems to me that it is a bad idea to provide deceptively simple looking functions that panic.
Panicing seems only reasonable if there's a way for the caller to check for simple preconditions that will cause a panic, but here whether fn from_read panics or not cannot reasonably be known at the callsite.