Add config build.errors.logLevelMissingShortcodeTemplate

[bep]

This would be especially handy when developing a new theme for some content, you would then say:

[build]
  [build.errors]
     logLevelMissingShortcodeTemplate="info"

Valid values would be debug, info, warn, error; default error.

[bep]

Well, thinking about this; It's a good idea, but for shortcodes with inner content we look at the template to know when to stop.