This is not quite finalized, but it appears that all errors will be reported as promise rejections of type DOMException (this includes WebIDL exceptions that will be converted into rejections). See <https://www.w3.org/Bugs/Public/show_bug.cgi?id=21740>.
It looks like that in the importKey method, there is an exception: https://www.w3.org/TR/WebCryptoAPI/#dfn-SubtleCrypto-method-importKey Will send an email to the working group to confirm the difference.
<rdar://problem/27858554>
bug 162010 is probably a duplicate of this one, or at least partially.
This bug should be resolved by the following revisions: <http://trac.webkit.org/changeset/207809>, <http://trac.webkit.org/changeset/208548>, <http://trac.webkit.org/changeset/208737>, <http://trac.webkit.org/changeset/208891>, <http://trac.webkit.org/changeset/208918>, <http://trac.webkit.org/changeset/209092>, <http://trac.webkit.org/changeset/209150>, <http://trac.webkit.org/changeset/209164>, <http://trac.webkit.org/changeset/209166>, <http://trac.webkit.org/changeset/209191>, <http://trac.webkit.org/changeset/209155>, <http://trac.webkit.org/changeset/209200>.