Handle exception when adding attachment that does not exist on file system. (#449)

src/Android/MainActivity.cs

@@ -203,6 +203,8 @@ namespace Bit.Android
                     return;
                 }
 
+                try
+                {
                     using(var stream = ContentResolver.OpenInputStream(uri))
                     using(var memoryStream = new MemoryStream())
                     {
@@ -211,6 +213,11 @@ namespace Bit.Android
                             new Tuple<byte[], string>(memoryStream.ToArray(), fileName ?? "unknown_file_name"));
                     }
                 }
+                catch (Java.IO.FileNotFoundException)
+                {
+                    return;
+                }
+            }
         }
 
         private void ListenYubiKey(bool listen)