Okay, the actual underlying mistake was pretty stupid, which made it

difficult to catch. This commit simplifies the solution.

What happened is that, for some idiotic reason, I was checking the type
of c.lastError, which was the previously set error for the context. This
was in place of doing the correct (read: smart) thing, which would have
been to check the type of the INCOMING error.

I'm not completely sure why I made this mistake, whether it was based
off of a previous incantation of SetError(), or because of some nested
dependencies that existed in a few ancient revisions, but it was still
something that should never have happened.

context.go

@@ -578,8 +578,7 @@ func (c *capstanContext) Error() string {
 
 // SetError changes the value of lastError for this context.
 func (c *capstanContext) SetError(err error) {
-	switch err.(type) {
-	case Context:
+	if _, ok := err.(Context); ok {
 		return
 	}
 	c.lastError = err