Improve location and text of error message

This program

mask = (b: true, false, true)
lenf(z) = sum(z) # type error
print(lenf(mask)) # error location should be here

produces this error message:

Type error: file.vm:2:15 --> z <--
Series "object" must be of type int or float

First, the location of the error should change to line 3 at lenf or mask. Second, the name object of the series is confusing. Ideally, it should be b or mask.