[nancy][grosminet] add cpu model + fix microarch + fix clock speed + better error message

data/grid5000/sites/nancy/clusters/grosminet/nodes/grosminet-1.json

@@ -334,8 +334,8 @@
     "turboboost_enabled": true
   },
   "performance": {
-    "core_flops": 33600000000,
-    "node_flops": 2419200000000
+    "core_flops": 83200000000,
+    "node_flops": 5990400000000
   },
   "processor": {
     "cache_l1": null,
@@ -343,10 +343,10 @@
     "cache_l1i": 32768,
     "cache_l2": 1048576,
     "cache_l3": 25952256,
-    "clock_speed": 2100000000,
+    "clock_speed": 2600000000,
     "ht_capable": true,
     "instruction_set": "x86-64",
-    "microarchitecture": "Zen 3",
+    "microarchitecture": "Cascade Lake-SP",
     "microcode": "0x5003604",
     "model": "Intel Xeon",
     "other_description": "Intel(R) Xeon(R) Gold 6240L CPU @ 2.60GHz",