Included error message if pyyaml is not installed on the system

oarstats.py

@@ -3,7 +3,12 @@
 
 import argparse
 import logging
+import sys
+try:
     import yaml
+except ImportError:
+    print("Package 'pyyaml' needs to be installed to be able to use this script!", file=sys.stderr)
+    exit(1)
 import subprocess
 from collections import defaultdict
 import re