expect_error does not return caught errors

[hughjonesd]

From ?expect_error:

If expect_error() captures an error, that is returned instead of the value.

In fact it always returns NULL on an error:

res <- expect_error(stop("foo"), "foo")
res
# NULL
res <- expect_error(stop("foo"))
res
# NULL
res <- expect_error(stop("foo"), NA)
Error: `stop("foo")` threw an error.
Message: foo
Class:   simpleError/error/condition
res
# NULL