Report handler panics through the logger and answer 500 (closes #187)

internal/middleware/middleware.go

@@ -133,16 +133,12 @@ const (
 	//   - The "log" delivery target, which exists to write the whole
 	//     inbound event to the log. Deliberate; see
 	//     internal/delivery/target_log.go.
-	//   - The widest line the service can write, which is neither an
-	//     access log line nor client-chosen. A handler panic arrives
-	//     through net/http's nil ErrorLog as one record carrying a
-	//     whole goroutine stack, above this figure — measured at
-	//     roughly 2,770 bytes. The exact width is not an invariant: it
-	//     moves with the goroutine number and with the source paths
-	//     baked into the stack. That it exceeds this ceiling does not
-	//     move. The value is the runtime's, not a client's; see
-	//     README.md and
-	//     https://git.eeqj.de/sneak/webhooker/issues/187.
+	//   - The record a recovered panic writes, which is not an access
+	//     log line: its client-supplied fields are charged the same
+	//     budgets, but it carries a whole goroutine stack as well and
+	//     is wider than this figure. It has its own stated ceiling,
+	//     MaxPanicLogLineBytes in recoverer.go, and is written once
+	//     per recovered panic rather than once per request.
 	MaxAccessLogLineBytes = 2560
 )