Propagate index errors in field_caps (#70245)

Currently we don't report any exceptions occuring during field_caps requests back to the user.
This PR adds a new failure section to the response which contains exceptions per index. 
In addition the response contains another field, `failed_indices`, with the number of indices that threw
an exception. If all of the requested indices fail, the whole request fails, otherwise the request succeeds 
and it is up to the caller to check for potential errors in the response body.

Closes #68994

qa/multi-cluster-search/src/test/resources/rest-api-spec/test/multi_cluster/30_field_caps.yml

@@ -61,10 +61,13 @@
   - is_false: fields.geo.keyword.on_aggregatable_indices
 
   - do:
+      catch: missing
       field_caps:
         index: 'my_remote_cluster:some_index_that_doesnt_exist'
         fields: [number]
-  - match: { 'fields': {} } # empty response - this index doesn't exists
+        
+  - match: { error.type: "index_not_found_exception" }
+  - match: { error.reason: "no such index [some_index_that_doesnt_exist]" }
 
   - do:
       field_caps:
@@ -86,6 +89,23 @@
   - match: {fields.number.keyword.aggregatable:         true}
   - match: {fields.number.keyword.type:                 keyword}
 
+  - do:
+      catch: bad_request
+      field_caps:
+        index: 'my_remote_cluster:field_caps_index_1'
+        fields: [number]
+        body:
+          runtime_mappings:
+            day_of_week:
+              type: keyword
+              script:
+                source: "bad syntax"
+
+  - match: { error.type: "script_exception" }
+  - match: { error.reason: "compile error" }
+  - match: { error.script : "bad syntax" }
+  - match: { error.lang : "painless" }
+
 ---
 "Get field caps from remote cluster with index filter":
   - skip: