tweak default messages for skip_if, skip_if_not

[kevinushey]

E.g.

> skip_if_not(FALSE)
Error: FALSE

It'd be nice if this showed something more like Error: FALSE is not TRUE or something with slightly more text.

As an example, the reported skip condition in devtools::test() is something like:

test-file.R:33: skip: we can do the thing
check_thing()

It would be a little more descriptive to instead write something like:

test-file.R:33: skip: we can do the thing
check_thing() is not TRUE

[hadley]

Yeah, agreed. Want to do a PR?

[hadley]

test-unnest.R:93: skip: unnesting zero row column preserves names
empty

Should be:

test-unnest.R:93: skipped: unnesting zero row column preserves names
Reason: empty