Refactoring, add mark error messages.

'{markName} and `{markName} return error messages if the markName does not
exist. This error message is shown in the status bar.
Linewise puts mostly implemented. Still buggy.

Sometimes {count} values get mixed up. To reproduce:
    y4j => Yanks 5 lines (expected)
    4yj => Yanks 5 lines in register r (expected)
    "ry4j => Yanks 5 lines in register r (expected)
    "r4yj => Yanks 17 lines into register r (unexpected)
Documented bug in TODO.
Fixed (operation|motion)Count bug in "x{count}4yj.

Parser was trying to parse next token as an operationCount or motionCount,
when it should have just been parsing it as a general "count" and then using
the logic defined in the tryToAssignOperationCount and
tryToAssignMotionCount functions to assign the count value to the
appropriate normalCommand attribute.