Better error message when nix-prefetch-url return an error 404

close nmattia#129
close nmattia#309
close nmattia#330

src/Niv/GitHub/Cmd.hs

@@ -11,6 +11,7 @@ module Niv.GitHub.Cmd
 where
 
 import Control.Applicative
+import Control.Monad
 import Data.Aeson ((.=))
 import qualified Data.Aeson as Aeson
 import Data.Bifunctor
@@ -160,7 +161,10 @@ nixPrefetchURL unpack packageName (T.unpack -> url) = do
   (exitCode, sout, serr) <- runNixPrefetch
   case (exitCode, lines sout) of
     (ExitSuccess, l : _) -> pure $ T.pack l
-    _ -> abortNixPrefetchExpectedOutput (T.pack <$> args) (T.pack sout) (T.pack serr)
+    _ -> do
+        let tserr = T.pack serr
+        checkNixPrefetchUrlNotFound (T.pack url) tserr
+        abortNixPrefetchExpectedOutput (T.pack <$> args) (T.pack sout) tserr
   where
     args = (if unpack then ["--unpack"] else []) <> [url, "--name", sanitizeName basename <> "-src"]
     runNixPrefetch = readProcessWithExitCode "nix-prefetch-url" args ""
@@ -172,6 +176,16 @@ nixPrefetchURL unpack packageName (T.unpack -> url) = do
     -- (note: we assume they don't begin with a period)
     isOk = \c -> isAlphaNum c || T.any (c ==) "+-._?="
 
+checkNixPrefetchUrlNotFound :: T.Text -> T.Text -> IO ()
+checkNixPrefetchUrlNotFound url serr = do
+    guard ("HTTP error 404" `T.isInfixOf` serr)
+    abort $ T.unlines
+        [ "The url '" <> url <> "' was not found by 'nix-prefetch-url'."
+        , "Changing your source's template or attributes may help you resolve the situation."
+        , "For further help, you can read the niv readme at https://github.com/nmattia/niv"
+        , "or open an ticket at https://github.com/nmattia/niv/issues/new."
+        ]
+
 abortNixPrefetchExpectedOutput :: [T.Text] -> T.Text -> T.Text -> IO a
 abortNixPrefetchExpectedOutput args sout serr =
   abort $