rc spell: Handle errors returned on the first line
rc spell: Handle errors returned on the first line

The first line returned by `aspell` isn't always an identification
string, it can also be an error.

This commit prevents the first line from being ignored in any case,
and allows errors to be reported consistently.

Related to #3330

mawww/kakoune,alexherbo2/kakoune,casimir/kakoune,jkonecny12/kakoune,Somasis/kakoune,Somasis/kakoune,casimir/kakoune,occivink/kakoune,lenormf/kakoune,casimir/kakoune,mawww/kakoune,Somasis/kakoune,occivink/kakoune,jkonecny12/kakoune,lenormf/kakoune,mawww/kakoune,danr/kakoune,mawww/kakoune,jkonecny12/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,Somasis/kakoune,alexherbo2/kakoune,occivink/kakoune,casimir/kakoune,danr/kakoune,alexherbo2/kakoune,danr/kakoune,occivink/kakoune,lenormf/kakoune,danr/kakoune,lenormf/kakoune