AGS: Notify user when adding a game that predates AGS 2.5

[Thunderforge]

ScummVM can only support games made with AGS 2.5 or later (internal version numbers 18 or later). However, ScummVM allows adding an older AGS game without issue (if its checksum is recognized) and the user is none the wiser that it is unsupported until they attempt to add it.

Current Behavior

1. On the ScummVM main screen, click "Add Game…"
2. Select a game built with a version of AGS earlier than 2.5 that also has a checksum recorded in ScummVM (such as ​robblanc1)
3. In the resulting dialog, click OK
4. Select the game on the list and click "Start"
5. Receive an error message saying that the game predates AGS 2.5

Desired Behavior

1. On the ScummVM main screen, click "Add Game…"
2. Select a game built with a version of AGS earlier than 2.5 that also has a checksum recorded in ScummVM (such as ​robblanc1)
3. A dialog appears saying that the game is not supported in ScummVM. The user can cancel the import or add it anyway (as they can with games that have unrecognized checksums).

This workflow informs the user earlier that the game is unsupported and thus both saves them time and eliminates the surprise that comes later that the game they imported won't work in ScummVM.

[dreammaster]

I've added a more explicit message trying to run pre-2.5 games