[pens] Improve error message if pen.moveTo() is omitted. (#1757)

Improve error message if pen.moveTo() is omitted. See also robotools/fontParts#470

Lib/fontTools/pens/pointPen.py

@@ -247,9 +247,11 @@ class SegmentToPointPen(AbstractPen):
 		self.contour.append((pt, "move"))
 
 	def lineTo(self, pt):
+		assert self.contour is not None, "contour missing required initial moveTo"
 		self.contour.append((pt, "line"))
 
 	def curveTo(self, *pts):
+		assert self.contour is not None, "contour missing required initial moveTo"
 		for pt in pts[:-1]:
 			self.contour.append((pt, None))
 		self.contour.append((pts[-1], "curve"))
@@ -257,12 +259,15 @@ class SegmentToPointPen(AbstractPen):
 	def qCurveTo(self, *pts):
 		if pts[-1] is None:
 			self.contour = []
+		else:
+			assert self.contour is not None, "contour missing required initial moveTo"
 		for pt in pts[:-1]:
 			self.contour.append((pt, None))
 		if pts[-1] is not None:
 			self.contour.append((pts[-1], "qcurve"))
 
 	def closePath(self):
+		assert self.contour is not None, "contour missing required initial moveTo"
 		if len(self.contour) > 1 and self.contour[0][0] == self.contour[-1][0]:
 			self.contour[0] = self.contour[-1]
 			del self.contour[-1]
@@ -276,6 +281,7 @@ class SegmentToPointPen(AbstractPen):
 		self.contour = None
 
 	def endPath(self):
+		assert self.contour is not None, "contour missing required initial moveTo"
 		self._flushContour()
 		self.contour = None