NoTagError is printed without ERROR prefix

[jasonkarns]

The NoTagError is categorized as an error and thus prevents printing of the shrinkwrap. But it is not printed to the user that the error is an ERROR, thus resulting in misleading output that there are no ERRORS. (This is probably not limited to NoTagError but is rather the fault of the default formatter not printing ERROR. https://github.com/uber/npm-shrinkwrap/blob/master/bin/formatters.js#L16)

this issue was uncovered through #99