improve error when error response is not JSON

docs/mithril.request.md

@@ -228,6 +228,23 @@ var file = m.request({
 
 ---
 
+### Using variable data formats
+
+By default, Mithril assumes both success and error responses are in JSON format, but some servers may not return JSON responses when returning HTTP error codes (e.g. 404)
+
+You can get around this issue by using `extract`
+
+```javascript
+var nonJsonErrors = function(xhr) {
+  return xhr.status > 200 ? JSON.stringify(xhr.responseText) : xhr.responseText
+}
+
+m.request({method: "GET", url: "/foo/bar.x", extract: nonJsonErrors})
+  .then(function(data) {}, function(error) {console.log(error)})
+```
+
+---
+
 ### Extracting Metadata from the Response
 
 The `extract` method can be used to read metadata from HTTP response headers or the status field of an XMLHttpRequest.

mithril.js

@@ -521,7 +521,8 @@ Mithril = m = new function app(window) {
 				deferred[e.type == "load" ? "resolve" : "reject"](response)
 			}
 			catch (e) {
-				if (e instanceof Error && e.constructor !== Error) throw e
+				if (e instanceof SyntaxError) throw new SyntaxError("Could not parse HTTP response. See http://lhorie.github.io/mithril/mithril.request.html#using-variable-data-formats")
+				else if (e instanceof Error && e.constructor !== Error) throw e
 				else deferred.reject(e)
 			}
 			if (xhrOptions.background !== true) m.endComputation()