repository: Push all usage of errors.Fatal out of the package

As the `Fatal` error type only includes a string, it becomes impossible
to inspect the contained error. This is for a example a problem for the
fuse implementation, which must be able to detect context.Canceled
errors.

Co-authored-by: greatroar <61184462+greatroar@users.noreply.github.com>

internal/repository/key.go

@@ -21,7 +21,7 @@ var (
 	ErrNoKeyFound = errors.New("wrong password or no key found")
 
 	// ErrMaxKeysReached is returned when the maximum number of keys was checked and no key could be found.
-	ErrMaxKeysReached = errors.Fatal("maximum number of keys reached")
+	ErrMaxKeysReached = errors.New("maximum number of keys reached")
 )
 
 // Key represents an encrypted master key for a repository.