check syntax does not group errors by file

[Curtis Hovey]

The recent change to append syntax errors in a background process add the file name/path for each error instead once. When checking one file, The file should only be listed once. When checking multiple files, each file should be listed once, and expanding the file will reveal the issues.

There are two issues with the code. The checker must set reporter.piter to None before each file to indicate that a new file is being appended. _message_file_lines() must append the file in real time. IT cannot queue the append because the piter must be known at that very moment to ensure all subsequent calls know their file.