Currently SRFI 170 requires that if its procedures fail, they must raise an exception, but provide no way to distinguish the condition object from all other kinds of condition objects. These procedures are available in the Chibi implementation, but aren't exposed. In IMHO decreasing order of importance, they are:
1) syscall-error? detects a SRFI 170 condition object and can be used in a guard-expression.
2) syscall-error:errno, syscall-error:message, syscall-error:procedure, syscall-error:data are accessors for the condition object. Note that syscall-error:procedure returns the actual procedure, not its name (as a symbol); this seems rather useless to me.
3) errno-error lets you create your own condition object and raise it immediately. It's an error to use this with an unknown errno, since it uses strerror() to convert from errno values to strings.
4) make-syscall-error lets you create your own condition object without raising it.
Scsh exposes only group 3. For the moment, I am adding groups 1 and 2, which are needed for error handling, and group 3, which is needed for error reporting in any future extensions to this SRFI.