check ycmd exit code

ycm-core/ycmd#467

lib/handler.coffee

@@ -66,10 +66,17 @@ launch = ->
         '--idle_suicide_seconds=600'
       ]
       options: {}
-      exit: (status) -> ycmdProcess = null
-    setTimeout(fulfill, 1000)
       stdout: (output) -> utility.debugLog 'CONSOLE', output
       stderr: (output) -> utility.debugLog 'CONSOLE', output
+      exit: (code) ->
+        ycmdProcess = null
+        switch code
+          when 3 then reject new Error 'Unexpected error while loading the YCM core library.'
+          when 4 then reject new Error 'YCM core library not detected; you need to compile YCM before using it. Follow the instructions in the documentation.'
+          when 5 then reject new Error 'YCM core library compiled for Python 3 but loaded in Python 2. Set the Python Executable config to a Python 3 interpreter path.'
+          when 6 then reject new Error 'YCM core library compiled for Python 2 but loaded in Python 3. Set the Python Executable config to a Python 2 interpreter path.'
+          when 7 then reject new Error 'YCM core library too old; PLEASE RECOMPILE by running the install.py script. See the documentation for more details.'
+    setTimeout fulfill, 1000
 
   Promise.all [findUnusedPort, generateRandomSecret, readDefaultOptions]
     .then processData