Handle connection loss more gracefully

[oskay]

If the USB cable is unplugged while plotting, one might get an error message like:

Failed after command: SM,30,333,-266
Failed after command: SM,30,265,-212
Failed after command: SM,30,200,-160
Failed after command: SM,30,133,-106
Failed after command: SM,30,66,-53
Error reading serial data.
Traceback (most recent call last):
  File "axidraw.py", line 2100, in <module>
    e.affect()
  File "/Applications/Inkscape_091.app/Contents/Resources/share/inkscape/extensions/inkex.py", line 285, in affect
    self.effect()
  File "axidraw.py", line 280, in effect
    self.plotDocument()
  File "axidraw.py", line 554, in plotDocument
    self.recursivelyTraverseSvg( self.svg, self.svgTransform )
  File "axidraw.py", line 625, in recursivelyTraverseSvg
    self.recursivelyTraverseSvg( node, matNew, parent_visibility=v )		
  File "axidraw.py", line 683, in recursivelyTraverseSvg
    self.plotPath( node, matNew )
  File "axidraw.py", line 1157, in plotPath
    self.plotSegmentWithVelocity( fX, fY, 0, 0)
  File "axidraw.py", line 1951, in plotSegmentWithVelocity
    if strButton[0] == '1': #button pressed
IndexError: string index out of range

This would be better served by a shorter error message of the form "Lost connectivity to AxiDraw. Did the USB cable get unplugged?"