export_error should warn if text match but class exists

[hadley]

If you call expect_error(error_with_class(), "text") testthat should warn that it's better to use class.

[gaborcsardi]

Hmmm, this will cause some damage I am afraid. But we'll see in the revdep checks....

[hadley]

It only generates a warning in the tests which doesn't affect R CMD check, so it should be ok.