added files out of sync error object

app/coffee/CompileController.coffee

@@ -4,6 +4,7 @@ Settings = require "settings-sharelatex"
 Metrics = require "./Metrics"
 ProjectPersistenceManager = require "./ProjectPersistenceManager"
 logger = require "logger-sharelatex"
+Errors = require "./Errors"
 
 module.exports = CompileController =
 	compile: (req, res, next = (error) ->) ->
@@ -15,7 +16,7 @@ module.exports = CompileController =
 			ProjectPersistenceManager.markProjectAsJustAccessed request.project_id, (error) ->
 				return next(error) if error?
 				CompileManager.doCompile request, (error, outputFiles = []) ->
-					if error?.message is "invalid state"
+					if error instanceOf Errors.FilesOutOfSyncError
 						code = 409 # Http 409 Conflict
 						status = "retry"
 					else if error?.terminated

app/coffee/Errors.coffee

@@ -5,6 +5,13 @@ NotFoundError = (message) ->
 	return error
 NotFoundError.prototype.__proto__ = Error.prototype
 
+FilesOutOfSyncError = (message) ->
+	error = new Error(message)
+	error.name = "FilesOutOfSyncError"
+	error.__proto__ = FilesOutOfSyncError.prototype
+	return error
+FilesOutOfSyncError.prototype.__proto__ = Error.prototype
 
 module.exports = Errors =
 	NotFoundError: NotFoundError
+	FilesOutOfSyncError: FilesOutOfSyncError

app/coffee/ResourceWriter.coffee

@@ -5,6 +5,7 @@ async = require "async"
 mkdirp = require "mkdirp"
 OutputFileFinder = require "./OutputFileFinder"
 Metrics = require "./Metrics"
+Errors = require "./Errors"
 logger = require "logger-sharelatex"
 settings = require("settings-sharelatex")
 
@@ -16,7 +17,7 @@ module.exports = ResourceWriter =
 		if request.syncType? is "incremental"
 			ResourceWriter.checkSyncState request.syncState, basePath, (error, ok) ->
 				logger.log syncState: request.syncState, result:ok, "checked state on incremental request"
-				return callback new Error("invalid state") if not ok
+				return callback new Errors.FilesOutOfSyncError("invalid state for incremental update") if not ok
 				ResourceWriter.saveIncrementalResourcesToDisk request.project_id, request.resources, basePath, callback
 		else
 			@saveAllResourcesToDisk request.project_id, request.resources, basePath, (error) ->