add warp-debug flag

for not it prints out the stat exception

warp/Network/Wai/Handler/Warp/Response.hs

@@ -215,7 +215,11 @@ sendRsp :: Connection
 sendRsp conn ver s0 hs0 restore (RspFile path mPart mRange hook) = restore $ do
     ex <- fileRange s0 hs path mPart mRange
     case ex of
-        Left _ -> sendRsp conn ver s2 hs2 id (RspBuilder body True)
+        Left _ex -> do
+#ifdef WARP_DEBUG
+          print _ex
+#endif
+          sendRsp conn ver s2 hs2 id (RspBuilder body True)
         Right (s, hs1, beg, len) -> do
             lheader <- composeHeader ver s hs1
             connSendFile conn path beg len hook [lheader]

warp/warp.cabal

@@ -31,6 +31,10 @@ Flag allow-sendfilefd
     Description: Allow use of sendfileFd (not available on GNU/kFreeBSD)
     Default:     True
 
+Flag warp-debug
+    Description: print debug output. not suitable for production
+    Default:     False
+
 Library
   Build-Depends:     base                      >= 3        && < 5
                    , array
@@ -77,6 +81,9 @@ Library
                      Network.Wai.Handler.Warp.Types
                      Paths_warp
   Ghc-Options:       -Wall
+
+  if flag(warp-debug)
+      Cpp-Options:   -DWARP_DEBUG
   if (os(linux) || os(freebsd) || os(darwin)) && flag(allow-sendfilefd)
       Cpp-Options:   -DSENDFILEFD
       Build-Depends: hashable