make storeKey throw exceptions

When storing content on remote fails, always display a reason why.

Since the Storer used by special remotes already did, this mostly affects
git remotes, but not entirely. For example, if git-lfs failed to connect to
the endpoint, it used to silently return False.

Remote/Adb.hs

@@ -160,7 +160,8 @@ adbSetup _ mu _ c gc = do
 store :: AndroidSerial -> AndroidPath -> Storer
 store serial adir = fileStorer $ \k src _p -> 
 	let dest = androidLocation adir k
-	in store' serial dest src
+	in unlessM (store' serial dest src) $
+		giveup "adb failed"
 
 store' :: AndroidSerial -> AndroidPath -> FilePath -> Annex Bool
 store' serial dest src = store'' serial dest src (return True)