Handle response payloads which are not JSON in generate_token.py (#2862)

Also, more gracefully handle payloads which are missing the authorization token.

Author: webbnh

lib/pbench/cli/agent/commands/generate_token.py

@@ -1,5 +1,7 @@
 """pbench-generate-token"""
 
+from json import JSONDecodeError
+
 import click
 import requests
 
@@ -30,8 +32,12 @@ def execute(self):
         except requests.exceptions.ConnectionError as exc:
             raise RuntimeError(f"Cannot connect to '{uri}'") from exc
 
-        payload = response.json()
-        if response.ok:
+        try:
+            payload = response.json()
+        except JSONDecodeError:
+            payload = {"message": response.text}
+
+        if response.ok and "auth_token" in payload:
             click.echo(payload["auth_token"])
             return 0