Error object: add type link

This enables the client to distinguish between different error conditions that share the same status code, by matching on the `type` URI. This is needed for the extension system, where the client needs to distinguish between the `profile` parameter not being supported by server, vs. the client providing insufficient profile extensions, both of which trigger a 415. This is the start of what I suggested in #683.
json-api · ethanresnick · Dec 26, 2015 · Dec 26, 2015 · Dec 26, 2015 · Dec 26, 2015
commit b09bc3db64406c94348404a4149e46a61cc9400b
diff --git a/_format/1.1/index.md b/_format/1.1/index.md
@@ -1768,6 +1768,10 @@ An error object **MAY** have the following members:
 * `links`: a [links object][links] containing the following members:
   * `about`: a [link][links] that leads to further details about this
     particular occurrence of the problem.
+  * <a id="error-objects-links-type"></a>`type`: a [link][link] whose URI
+    identifies the type of error that this particular error is an instance of.
+    When dereferenced, this URI **SHOULD** provide a human-readable explanation
+    of the general error.
 * `status`: the HTTP status code applicable to this problem, expressed as a
   string value.
 * `code`: an application-specific error code, expressed as a string value.