Added check for invalid creds

Without the added check, the script will error out on line 29 since the key doesn't exist in the dict.  This at least gives a reason.

examples/proofpoint_tap.py

@@ -22,6 +22,9 @@ headers = {
 }
 
 responseSiem = requests.request("GET", urlSiem, headers=headers, params=queryString)
+if 'Credentials authentication failed' in str(responseSiem.text):
+    print("Credentials invalid, please edit keys.py and try again")
+    quit()
 
 jsonDataSiem = json.loads(responseSiem.text)