Handle errors better in KDevFormatFile::executeCommand()

• Don't try to execute an empty command.
• Make error messages refer to a command-line interpreter rather than to the command passed to it. Note that kdev_format_source did not detect when a user-specified command failed to start or crashed.
• Warn when the command-line interpreter exits with a non-zero code. A non-zero exit code likely means that the process executed by the interpreter failed to start, crashed or reported failure. It can also be a nonfatal error report from the user-supplied command. Therefore let kdev_format_source report it as a warning, not an error.