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.

Types/Remote.hs

@@ -84,7 +84,8 @@ data RemoteA a = Remote
 	-- Transfers a key's contents from disk to the remote.
 	-- The key should not appear to be present on the remote until
 	-- all of its contents have been transferred.
-	, storeKey :: Key -> AssociatedFile -> MeterUpdate -> a Bool
+	-- Throws exception on failure.
+	, storeKey :: Key -> AssociatedFile -> MeterUpdate -> a ()
 	-- Retrieves a key's contents to a file.
 	-- (The MeterUpdate does not need to be used if it writes
 	-- sequentially to the file.)

Types/StoreRetrieve.hs

@@ -23,7 +23,7 @@ isByteContent (FileContent _) = False
 
 -- Action that stores a Key's content on a remote.
 -- Can throw exceptions.
-type Storer = Key -> ContentSource -> MeterUpdate -> Annex Bool
+type Storer = Key -> ContentSource -> MeterUpdate -> Annex ()
 
 -- Action that retrieves a Key's content from a remote, passing it to a
 -- callback, which will fully consume the content before returning.