Tools: use Python 3, improve import checks

In more detail:
- Change shebang to Python 3.
- Suggest installation using pip3 as user.

Tools/validate_yaml.py

@@ -9,19 +9,23 @@ import sys
 
 try:
     import yaml
-except:
-    print("Failed to import yaml.")
-    print("You may need to install it with 'sudo pip install pyyaml'")
+except ImportError as e:
+    print("Failed to import yaml: " + e)
     print("")
-    raise
+    print("You may need to install it using:")
+    print("    pip3 install --user pyyaml")
+    print("")
+    sys.exit(1)
 
 try:
     import cerberus
-except:
-    print("Failed to import cerberus.")
-    print("You may need to install it with 'sudo pip install cerberus'")
+except ImportError as e:
+    print("Failed to import cerberus: " + e)
     print("")
-    raise
+    print("You may need to install it using:")
+    print("    pip3 install --user cerberus")
+    print("")
+    sys.exit(1)
 
 
 parser = argparse.ArgumentParser(description='Validate YAML file(s) against a schema')