Add exception tests

Author: bunnymatic

exercises/nucleotide-count/canonical-data.json

@@ -25,6 +25,14 @@
         "strand": "CGATTGGG",
         "nucleotide": "T",
         "expected": 2
+      },
+      {
+        "description": "validates nucleotides in requested nucleotide",
+        "strand": "GACT",
+        "nucleotide": "X",
+        "expected": {
+          "error": "Invalid requested nucleotide"
+        }
       }
     ]
   },
@@ -60,6 +68,14 @@
           "G": 17,
           "T": 21
         }
+      },
+      {
+        "description": "validates nucleotides in the strand",
+        "strand": "AGXXACT",
+        "nucleotide": "A",
+        "expected": {
+          "error": "Invalid nucleotide in strand"
+        }
       }
     ]
   }