feat(logging): make error handling configurable

- user can set a flag "fatals" in his grunt config to true
- if set, a grunt.warn is thrown on error (which can be overriden with --force)
fix(logging): correct output of completed function
fix(errors): let null pass as no error
docs(README): add note to error handling from cli
docs(README): correct line break
chore(task): remove unneccessary error output
Merge pull request #9 from h-evers/master

make error handling configurable