so-telegraf-cred: make executable and harden error handling

so-telegraf-cred was committed with mode 644, causing
`so-telegraf-cred add "$MINION_ID"` in so-minion's add_telegraf_to_minion
to fail with "Permission denied" and log "Failed to provision postgres
telegraf cred for <minion>". Mark it executable.

Also bail early in seed_creds_file if mkdir/printf/chmod fail, and in
so-yaml.py loadYaml surface a clear stderr message with the filename
instead of an unhandled FileNotFoundError traceback.

salt/manager/tools/sbin/so-yaml.py

@@ -39,9 +39,16 @@ def showUsage(args):
 
 
 def loadYaml(filename):
-    file = open(filename, "r")
-    content = file.read()
-    return yaml.safe_load(content)
+    try:
+        with open(filename, "r") as file:
+            content = file.read()
+            return yaml.safe_load(content)
+    except FileNotFoundError:
+        print(f"File not found: {filename}", file=sys.stderr)
+        sys.exit(1)
+    except Exception as e:
+        print(f"Error reading file {filename}: {e}", file=sys.stderr)
+        sys.exit(1)
 
 
 def writeYaml(filename, content):