Remove the -count=94919 workaround for :AppendTabSession.

As the command only takes the optional count _before_ the command (like :tabnew, as per the documentation), no count can be detected by -1 if we specify -range=-1.
This also avoids the unexpected behavior of -count that a passed argument in the form "42foo" is parsed as count=42 and args=foo. With this change, one can properly define session names that start with numbers.
Don't leave behind empty tab page when :AppendTabSession is unsuccessful.

Make xolox#session#open_cmd() indicate the success (and whether a warning message was given) via its return value. Check that in xolox#session#append_tab_cmd() and undo the opening of the new tab page in case no session was opened.
Merge pull request #82: Remove -count=94919 workaround for :AppendTabSession
Merge pull request #83: Don't leave behind empty tab page when :AppendTabSession is unsuccessful