Add protection against syncing non place projects (#691)

Closes #113.

Does what it says on the tin. When you try to sync a non-place project,
it gives a clear message instead of crashing.

plugin/src/ServeSession.lua

@@ -232,6 +232,22 @@ function ServeSession:__initialSync(serverInfo)
 				Log.error("Could not compute a diff to catch up to the Rojo server: {:#?}", catchUpPatch)
 			end
 
+			for _, update in catchUpPatch.updated do
+				if
+					update.id == self.__instanceMap.fromInstances[game]
+					and update.changedClassName ~= nil
+				then
+					-- Non-place projects will try to update the classname of game from DataModel to
+					-- something like Folder, ModuleScript, etc. This would fail, so we exit with a clear
+					-- message instead of crashing.
+					return Promise.reject(
+						"Cannot sync a model as a place."
+						.. "\nEnsure Rojo is serving a project file that has a DataModel at the root of its tree and try again."
+						.. "\nSee project file docs: https://rojo.space/docs/v7/project-format/"
+					)
+				end
+			end
+
 			Log.trace("Computed hydration patch: {:#?}", debugPatch(catchUpPatch))
 
 			local userDecision = "Accept"