They give output like this:
CONSOLE MESSAGE: line 16: TYPE_MISMATCH_ERR: DOM Exception 17: The type of an object was incompatible with the expected type of the parameter associated to the object.
Committed r59872: <http://trac.webkit.org/changeset/59872>
Accidentally closed this bug via webkit-patch.
*** This bug has been marked as a duplicate of bug 39434 ***