vcs-svn: allow input errors to be detected promptly

vcs-svn: allow input errors to be detected promptly

The line_buffer library silently flags input errors until
buffer_deinit time; unfortunately, by that point usually errno is
invalid.  Expose the error flag so callers can check for and
report errors early for easy debugging.

	some_error_prone_operation(...);
	if (buffer_ferror(buf))
		return error("input error: %s", strerror(errno));

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>