made failure reason ob readLoop more obvious

src/Network/Gemini/Capsule/Internal.hs

@@ -123,7 +123,7 @@ readLoop maxLen src =
     Just bs -> do
       let len = BS.length bs
       when (len > maxLen) $
-        MaybeT $ return Nothing
+        fail "maximum length exceeded"
       (byteString bs <>) <$> readLoop (maxLen - len) src
 
 --jl