Fixes #435, fixes #436

Sync uses run interface to execute commands. Run scans given io.Reader and creates a command from each line. If sync finds multiple files to remove, generated command string exceeds the maximum token size of bufio.Scanner and we are getting bufio.Scanner: token too long errors. As described in the issues, sync fails if it tries to remove a large number of files.

Official go documentation recommends using bufio.Reader for handling large tokens:

Programs that need more control over error handling or large tokens, or must run sequential scans 
on a reader, should use bufio.Reader instead.

This PR fixes two bugs:

1. Currently, scanner errors are ignored, and commands silently fail if we encounter scanner errors. Always print scanner errors before exiting the command.

2. Use bufio.Reader instead of bufio.Scanner to handle large tokens.