pascals-triangle: implement error object

per exercism#1311
pascals-triangle: update comment re: errors

Per @Insti's suggestion....

exercises/pascals-triangle/canonical-data.json

@@ -1,10 +1,10 @@
 {
   "exercise": "pascals-triangle",
-  "version": "1.3.0",
+  "version": "1.4.0",
   "comments": [
     "Expectations are represented here as an array of arrays.",
     "How you represent this idiomatically in your language is up to you.",
-    "An expectation of -1 indicates some sort of failure should occur"
+    "An expectation of 'error' indicates some sort of failure should occur."
   ],
   "cases": [
     {
@@ -80,7 +80,7 @@
           "input": {
             "count": -1
           },
-          "expected": -1
+          "expected": {"error": "count must be positive"}
         }
       ]
     }