Since this was done with extract there really isn't anyway to have a lower than expected OG short of watering your beer down too much, a severe boilover, or not adding all the extract. If you followed the recipe correctly then you'd started out at the right spot - 1.050 is fine for a stout.. they tend to be lower on the ABV (except for imperial stouts of course).
What you're confusing is Final Gravity and Alcohol By Volume. Final gravity just tells you what the density of your beer is when fermentation has ceased. Remember that water is the reference point - 60 deg water will read as 1.000 on your hydrometer. Sugar is thicker than water and alcohol is thinner, so the Specific Gravity before fermentation will be high - usually upwards of 1.045. Because the thick sugar is replaced with alcohol the finished beer will be much 'thinner', almost as thin as water. When the airlock stops bubbling then the yeast have basically done what they're gonna do - this is when you take a final gravity reading to see how much of the sugars the yeast have fermented into alcohol. The higher the Original Gravity and the lower the Final Gravity (the bigger the gap between the two) the more alcohol you've got. Thus, a *lower* final gravity typically means a *higher* alcohol content. You're problem here is a higher than expected final gravity giving you a lower than anticipated alcohol content.
So, if the airlock truly has stopped bubbling and the fermentation has ceased then you have a problem. Fermentation usually does not stop until around 1.015 or less.. if it stops higher then this then you have what is called a "stuck fermentation". The yeast basically lost interest and stopped fermenting when there's still fermentable sugar left in the wort/beer. You probably also noticed that the beer tastes a bit sweet - this is because there is still quite a bit of sugar left in the beer that needs to be fermented out. What you'll need to do is get another packet of yeast (assuming you're using dried yeast), rehydrate it, and pitch it into the beer BUT DO NOT AERATE! Hopefully the fermentation should pick up again in a day or two. Wait until it completes and bottle as usual.
or
If the fermentation has not ceased and you're just checking the Specific Gravity to see how things are coming then you just need to wait a few more days until the beer hits that 1.015 mark... at this point it is not ready to be bottled as it has not reached the correct Final Gravity
edit: I see you already bottled it.. ah well too late now. Let us know how it turns out. If the fermentation unsticks then you may have bottle bombs on your hands.. watch out. On the other hand, if the beer doesn't unstick then I suppose it won't carbonate and you'll have flat semi-sweet beer. Let's hope for the former.. but heck I'm sure it'll be drinkable either way. And if it sat that long in the secondary then I'll bet the fermentation just stopped short.. disregard that third paragraph there.
