fs: Quietly ignore non-dir targets in get_files_in_dir() (BFS only)

The non-BFS implementation already ignored this case without making
noise in stderr, and this function is used by delete_directory(), which
also accepts both regular files and directories. The add-ons management
code has always relied on this behavior.

src/filesystem.cpp

@@ -138,6 +138,7 @@ void get_files_in_dir(const std::string &directory,
 	DIR* dir = opendir(directory.c_str());
 
 	if(dir == NULL) {
+		// Probably not a directory, let the caller deal with it.
 		return;
 	}
 

src/filesystem_boost.cpp

@@ -335,7 +335,7 @@ void get_files_in_dir(const std::string &dir,
 	bfs::directory_iterator di(dirpath, ec);
 	bfs::directory_iterator end;
 	if (ec) {
-		ERR_FS << "Failed to open directory " << dirpath.string() << ": " << ec.message() << '\n';
+		// Probably not a directory, let the caller deal with it.
 		return;
 	}
 	for(; di != end; ++di) {