Calculate does not always return an error for incorrect plot statements

[greenfeld]

Calculate does not always return an error for incorrect plot statements. This has been seen on various OLPC and SoaS builds up to the latest F14 daily builds with Calculate versions 33 & 34.

For instance, the following calculate statement lines will not return errors or do anything noticeable in the GUI once "Enter" is pressed:

plot(x*x,x=-5,5) # Should be plot(x*x,x=-5..5)
plot(x=-5..5,x*x) # Reversed arguments

[greenfeld]

Non-plot statements like "2=4" (should be 2==4) may result in no error message either.

[lionaneesh]

Fix @ ​http://git.sugarlabs.org/calculate/mainline/merge_requests/5