Provide position for unimplemented augmented assignment

Avoids having "dummy position" appear in the resulting error message.

Also call "convert" on the left- and right-hand sides like we do in
the other cases.

M2/Macaulay2/d/convertr.d

@@ -298,7 +298,7 @@ export convert0(e:ParseTree):Code := (
 	      is t:Token do Code(augmentedAssignmentCode(
 		      b.Operator.entry, convert(b.lhs), convert(b.rhs), t.entry, pos))
 	      else Code(augmentedAssignmentCode(
-		      b.Operator.entry, dummyCode, dummyCode, dummySymbol, dummyPosition)) -- CHECK
+		      b.Operator.entry, convert(b.lhs), convert(b.rhs), dummySymbol, pos))
 		      )
 	  else Code(binaryCode(b.Operator.entry.binary, convert(b.lhs), convert(b.rhs), pos))
 	  )