Replace Runner-specific LinterError with general one

Commit a40511f introduced a new `LinterError` class under the new
`SCSSLint::Exceptions` namespace. Replace the one-off `LinterError` in
`Runner` with this more general one.

This brings us closer towards having all our custom exceptions defined
in one place.

Change-Id: If2803b35c2313264079e2541d4724d9cb0033110
Reviewed-on: http://gerrit.causes.com/40837
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>