Change error behavior in move_cursor + bug fix.
Change error behavior in move_cursor + bug fix.

Instead of silently "failing" when an invalid index is passed to move_cursor we
fail fast by raising an exception. Also fixed a related bug in that the
doc_length should be initialized to 1, since that is the length of the start
delta.

fraywing/quill,tim-mc/quill,jziggas/quill,kmoe/quill,syungkeaw/quill,psiemens/quill,hampsterx/quill,casetext/quill,dahal/quill,DemocracyOS/quill,willrowe/quill,quilljs/quill,sferoze/quilljs,sferoze/quilljs,willrowe/quill,DemocracyOS/quill,sferoze/quilljs,smartsocialmedia/quill,studio666/quill,Asana/quill-1,KentZheng/gitFlowTIA,Asana/quill-1,JoshBarr/quill,KentZheng/gitFlowTIA,voxmedia/quill,kmoe/quill,KentZheng/testingflow,KentZheng/gitFlowTIB,sferoze/quilljs,KentZheng/testingflow,eXcomm/quill,voxmedia/quill,quilljs/quill,milqfm/quill,KentZheng/gitFlowTIB,psiemens/quill,gutenye/quill,quilljs/quill,mobitroll/kahoot-quill,milqfm/quill,willrowe/quill,fraywing/quill,tim-mc/quill,anovi/quill,jcppman/quill,casetext/quill,smartsocialmedia/quill,Benibur/quill,mobitroll/kahoot-quill,praneybehl/quill,voxmedia/quill,jayzawrotny/quill