Improve error handling for poorly formed relationships

Tells the user what they did wrong when they forget to wrap their
relationship changes in a `{ data: ... }` object

src/steps/pre-query/parse-request-primary.js

@@ -22,14 +22,24 @@ export default function(data, parseAsLinkage) {
     }
 
     catch (error) {
-      const title = "The resources you provided could not be parsed.";
-      const details = `The precise error was: "${error.message}".`;
-      reject(new APIError(400, undefined, title, details));
+      if (error instanceof APIError) {
+        reject(error);
+      }
+
+      else {
+        const title = "The resources you provided could not be parsed.";
+        const details = `The precise error was: "${error.message}".`;
+        reject(new APIError(400, undefined, title, details));
+      }
     }
   });
 }
 
-function relationshipObjectFromJSON(json) {
+function relationshipObjectFromJSON(json, relationship) {
+  if (typeof json.data === "undefined") {
+    throw new APIError(400, undefined, `No data key was found for the '${relationship}' relationship.`);
+  }
+
   return new RelationshipObject(linkageFromJSON(json.data));
 }
 
@@ -42,7 +52,7 @@ function resourceFromJSON(json) {
 
   //build RelationshipObjects
   for(let key in relationships) {
-    relationships[key] = relationshipObjectFromJSON(relationships[key]);
+    relationships[key] = relationshipObjectFromJSON(relationships[key], key);
   }
 
   return new Resource(json.type, json.id, json.attributes, relationships, json.meta);