Errors during rubocop linting should return non-zero exit code

[oehlschl]

Filing this after #286, in which we didn't notice a breaking rubocop upgrade in our CI because erblint returns a 0 exit code in the case that the rubocop linter fails in error.

It looks like this could be solved by either:

• updating the rubocop linter to be aware of .errors in addition to .offenses
• instantiating the rubocop Team with the raise_cop_error option

The latter works as expected and prints a backtrace (it gets caught by this line), though the output isn't particularly pretty.