replace dumb printf-style syntax with str.format

Signed-off-by: Kwpolska <kwpolska@gmail.com>
Update nikola/plugins/command_check.py
typo

Signed-off-by: Kwpolska <kwpolska@gmail.com>
Merge branch 'better-errors-no-percent' of github.com:Kwpolska/nikola into better-errors-no-percent
Merge branch 'master' of https://github.com/ralsina/nikola into better-errors-no-percent

Signed-off-by: Kwpolska <kwpolska@gmail.com>

Conflicts (already resolved):
	nikola/plugin_categories.py
	nikola/plugins/command_bootswatch_theme.py
	nikola/plugins/command_build.py
	nikola/plugins/command_check.py
	nikola/plugins/command_deploy.py
	nikola/plugins/command_import_blogger.py
	nikola/plugins/command_install_theme.py
	nikola/plugins/command_new_post.py
	nikola/plugins/command_serve.py
	nikola/plugins/task_redirect.py
missed this one

Signed-off-by: Kwpolska <kwpolska@gmail.com>
Merge pull request #326 from Kwpolska/better-errors-no-percent

Make errors more informative; replace dumb printf-style syntax with not-so-new str.format() hotness