[varLib.avarPlanner] A couple of error messages

Lib/fontTools/varLib/avarPlanner.py

@@ -62,7 +62,8 @@ def planWeightAxis(
     if "avar" in font:
         log.debug("Checking that font doesn't have weight mapping already.")
         existingMapping = font["avar"].segments["wght"]
-        assert not existingMapping or existingMapping == {-1: -1, 0: 0, +1: +1}
+        if existingMapping and existingMapping != {-1: -1, 0: 0, +1: +1}:
+            log.error("Font already has a `avar` weight mapping. Remove it.")
 
     out = {}
     outNormalized = {}
@@ -160,6 +161,9 @@ def main(args=None):
     )
 
     font = TTFont(options.font)
+    if not "fvar" in font:
+        log.error("Not a variable font.")
+        sys.exit(1)
     fvar = font["fvar"]
     wghtAxis = slntAxis = None
     for axis in fvar.axes:
@@ -170,10 +174,12 @@ def main(args=None):
 
     if "avar" in font:
         existingMapping = font["avar"].segments["wght"]
+        if wghtAxis:
             font["avar"].segments["wght"] = {}
     else:
         existingMapping = None
 
+    if wghtAxis:
         out, outNormalized = planWeightAxis(
             font, wghtAxis.minValue, wghtAxis.defaultValue, wghtAxis.maxValue
         )
@@ -192,6 +198,7 @@ def main(args=None):
     if "avar" not in font:
         font["avar"] = newTable("avar")
     avar = font["avar"]
+    if wghtAxis:
         avar.segments["wght"] = outNormalized
 
     designspaceSnippet = (