Fix #25

Brackets doesn't provide file contents (._contents property) if the files are not in a watched directory. This fixes #25 by providing an instruction to the user instead of silently failing to show the comparison.

src/main.js

@@ -100,6 +100,12 @@ define(function (require, exports, module) {
         return; 
       }
 
+      if (mFile._contents === null || oFile._contents === null) {
+        notifier.error('To compare files, please open the folder containing these files in Brackets using the pane on the left.');
+        Logger.error('To compare files, please open the folder containing these files in Brackets using the pane on the left.');
+        return; 
+      }
+
       if (getExtension(mFile._name) !== getExtension(oFile._name)) {
         notifier.error('Cannot compare files of different types. Please select files of the same type.');
         Logger.error('Cannot compare files of different types. Please select files of the same type.');
@@ -152,4 +158,4 @@ define(function (require, exports, module) {
     // Create a new panel to hold the diff views
     comparePanel = WorkspaceManager.createBottomPanel("compare.panel", $(COMPARE_PANEL_HTML), 1000);
   });
-});
+});