improve error message when special remote program cannot be run

Remote/External.hs

@@ -369,12 +369,8 @@ startExternal :: ExternalType -> Annex ExternalState
 startExternal externaltype = do
 	errrelayer <- mkStderrRelayer
 	liftIO $ do
-		(Just hin, Just hout, Just herr, pid) <- createProcess $
-			(proc cmd [])
-				{ std_in = CreatePipe
-				, std_out = CreatePipe
-				, std_err = CreatePipe
-				}
+		(Just hin, Just hout, Just herr, pid) <- 
+			createProcess p `catchIO` runerr
 		fileEncoding hin
 		fileEncoding hout
 		fileEncoding herr
@@ -390,6 +386,13 @@ startExternal externaltype = do
 			}
   where
 	cmd = externalRemoteProgram externaltype
+	p = (proc cmd [])
+		{ std_in = CreatePipe
+		, std_out = CreatePipe
+		, std_err = CreatePipe
+		}
+
+	runerr _ = error ("Cannot run " ++ cmd ++ " -- Make sure it's in your PATH and is executable.")
 
 	checkearlytermination Nothing = noop
 	checkearlytermination (Just exitcode) = ifM (inPath cmd)

doc/special_remotes/ipfs/comment_6_8621bbd87982f73cf401cfa09d95d1e4._comment

@@ -0,0 +1,10 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 6"""
+ date="2015-11-18T16:22:51Z"
+ content="""
+@rob.syme I think that error message is pretty clear: git-annex-remote-ipfs
+is not present in your PATH or perhaps is not executable.
+
+(Still, the error message could be improved a bit, doing that now.)
+"""]]