bake: Several bug taxonomy-related fixes for incorrect incremental bakes. * Improve how the baker processes taxonomy terms and figures out what needs to be re-baked or not. * Create bake entries for clean taxnomy terms so they're not deleted by an incremental bake. * Add more information to bake records. * Slugify taxonomy terms is now done by the route in one place. * Fix a bug where the cache key for invalidating rendered segments was not computed the same way as when the caching was done. * Fix how term combinations are passed around, rendered, printed, parsed, etc. (TODO: more word needed in the routing functions) * Expose to the template whether a taxonomy term is a combination or not. * Display term combinations better in the built-in theme. * Rename `route.taxonomy` to `route.taxonomy_name` to prevent confusion. * Add options to show bake records for previous bakes.

class TemplateNotFoundError(Exception):
    pass


class TemplatingError(Exception):
    def __init__(self, message, filename=None, lineno=-1):
        super(TemplatingError, self).__init__()
        self.message = message
        self.filename = filename
        self.lineno = lineno

    def __str__(self):
        msg = ''
        if self.filename:
            msg += self.filename
        if self.lineno >= 0:
            msg += ', line %d' % self.lineno
        msg += ': ' + self.message
        return msg


class TemplateEngine(object):
    EXTENSIONS = []

    def initialize(self, app):
        self.app = app

    def renderString(self, txt, data, filename=None, line_offset=0):
        raise NotImplementedError()

    def renderFile(self, paths, data):
        raise NotImplementedError()