Fixed https://github.com/olihel/grunt-fetch-pages/issues/1

If followLinks is true a branch is executed that relies on the existance of baseURL, whichs existance in combination with followLinks was not checked for.

Old behaviour:
- if followLinks was true (default), but no baseURL was set: Fatal error: Parameter 'url' must be a string, not undefined

New behaviour:
- if followLinks is true (default), but no baseURL was set: >> baseURL is mandatory if followLinks is true. Either set followLinks to false or define baseURL.

If grunt is present a grunt.log.error is triggered; else console.error is used.