Fix #579: Pattern match failure on missing ipython binary

Change error message from an uncaught exception to an actual error
message.

src/IHaskell/IPython.hs

@@ -72,6 +72,13 @@ ipythonCommand = do
       Nothing -> "ipython"
       Just _  -> "jupyter"
 
+locateIPython :: SH.Sh SH.FilePath
+locateIPython = do
+  mbinary <- SH.which "ipython"
+  case mbinary of
+    Nothing      -> SH.errorExit "The IPython binary could not be located"
+    Just ipython -> return ipython
+
 -- | Run the IPython command with any arguments. The kernel is set to IHaskell.
 ipython :: Bool         -- ^ Whether to suppress output.
         -> [Text]       -- ^ IPython command line arguments.
@@ -201,15 +208,17 @@ installKernelspec replace opts = void $ do
       src <- liftIO $ Paths.getDataFileName $ "html/" ++ file
       SH.cp (SH.fromText $ T.pack src) (tmp SH.</> kernelName SH.</> file)
 
-    Just ipython <- SH.which "ipython"
+    ipython <- locateIPython
+
     let replaceFlag = ["--replace" | replace]
         installPrefixFlag = maybe ["--user"] (\prefix -> ["--prefix", T.pack prefix]) (kernelSpecInstallPrefix opts)
         cmd = concat [["kernelspec", "install"], installPrefixFlag, [SH.toTextIgnore kernelDir], replaceFlag]
+
     SH.silently $ SH.run ipython cmd
 
 kernelSpecCreated :: SH.Sh Bool
 kernelSpecCreated = do
-  Just ipython <- SH.which "ipython"
+  ipython <- locateIPython
   out <- SH.silently $ SH.run ipython ["kernelspec", "list"]
   let kernelspecs = map T.strip $ T.lines out
   return $ T.pack kernelName `elem` kernelspecs