LocateUri: migrate from class Error to C++ exceptions

test/test_translate_song.cxx

@@ -142,13 +142,13 @@ Client::GetStorage() const
 	return ::storage;
 }
 
-bool
-Client::AllowFile(gcc_unused Path path_fs, gcc_unused Error &error) const
+void
+Client::AllowFile(gcc_unused Path path_fs) const
 {
-	/* always return false, so a SongLoader with a non-nullptr
+	/* always fail, so a SongLoader with a non-nullptr
 	   Client pointer will be regarded "insecure", while one with
 	   client==nullptr will allow all files */
-	return false;
+	throw std::runtime_error("foo");
 }
 
 static std::string