Fix exceptions in SentenceMotion and ParagraphMotion

(1) I added line index checking in TestTextContent.getLineInformation(), now it throws an exception when the line is out of range. This mimics the actual behavior of the EclipseTextContent. Without this check, exceptions could be thrown when the plugin was enabled in Eclipse even though unit tests were ok.

(2) I tried to correct the line index checking in SentenceMotion and ParagraphMotion. Without this correction, exceptions were thrown when cursor was on the last line even though the unit tests were ok (actually this is the reason why I did the (1) fix).