changed error for is/2

Hi,

In running the tests for the main branch 5.4, I noticed that Built_In's built_in
test failed. This seems to be because of a changed error for something like:

atom is 1 + 1.

In 5.3, we get:

[eclipse 3]: an_atom is 1 + 1.
number expected in +(1, 1, an_atom)

In 5.4 (I have updated and rebuilt my main branch just now), we get:

Version 5.4 #19, Sat Apr  6 21:32 2002
[eclipse 1]: a is 1 + 1.
type error in +(1, 1, a)

I assume this is due to some recent changes? The Built_In tests should be updated,
I guess? (My built_in.ref seems to be current)

Cheers,

Kish