Context: especially since we're moving almost all process related procedures to a future SRFI focused on doing those sorts of things safely, we're going to create a separate error reporting SRFI, like the in progress timespec SRFI.
From my experiences in creating a Chibi Scheme implementation of SRFI-170, all optional string arguments to whatever is called and how it's displayed:
Location: could be filled in with an automated file name and line number facility if the Scheme implementation supports that, or just could be supplied by the author, like "in procedure X, in the argument checking block".
Reason: while overloading errno/inval for invalid arguments is not the worst thing we're doing, and I assume we'll break out our own version of that, I'd like to be able to be more specific and say things like "The first argument must be a port" or "The first argument must be a terminal input port."
POSIX call: if an error comes directly from a POSIX call, I'd like to be able to supply the name like "lstat", and have the SRFI handle how it's decorated, like adding a "while calling " suffix.