webapp: detect on first run if git is not installed, and tell the user, rather than crashing non-informatively

Mostly for Windows, which has no sane package management..

Assistant/WebApp/Configurators/Local.hs

@@ -160,6 +160,8 @@ getFirstRepositoryR :: Handler Html
 getFirstRepositoryR = postFirstRepositoryR
 postFirstRepositoryR :: Handler Html
 postFirstRepositoryR = page "Getting started" (Just Configuration) $ do
+	unlessM (liftIO $ inPath "git") $
+		error "You need to install git in order to use git-annex!"
 #ifdef __ANDROID__
 	androidspecial <- liftIO $ doesDirectoryExist "/sdcard/DCIM"
 	let path = "/sdcard/annex"

Command/WebApp.hs

@@ -163,7 +163,8 @@ firstRun listenhost = do
 			hFlush stdout
 			go
 		| otherwise = do
-			browser <- maybe Nothing webBrowser <$> Git.Config.global
+			browser <- maybe Nothing webBrowser
+				<$> catchDefaultIO Nothing Git.Config.global
 			openBrowser browser htmlshim url Nothing Nothing
 			go
 	  where