[remote asset api] mention RESOURCE_EXHAUSTED with other error status codes (#239)

This status value was already mentioned in the Fetch service comments,
but was not included in the FetchBlobResponse or FetchDirectoryResponse
status field comments alongside some other similar values. Let's make
this more clear.

Author: mostynb

build/bazel/remote/asset/v1/remote_asset.pb.go

@@ -216,6 +216,8 @@ message FetchBlobResponse {
   //   requested an asset from a disallowed origin.
   // * `ABORTED`: The operation could not be completed, typically due to a
   //   failed consistency check.
+  // * `RESOURCE_EXHAUSTED`: There is insufficient quota of some resource to
+  //   perform the requested operation. The client may retry after a delay.
   google.rpc.Status status = 1;
 
   // The uri from the request that resulted in a successful retrieval, or from
@@ -295,6 +297,8 @@ message FetchDirectoryResponse {
   //   requested an asset from a disallowed origin.
   // * `ABORTED`: The operation could not be completed, typically due to a
   //   failed consistency check.
+  // * `RESOURCE_EXHAUSTED`: There is insufficient quota of some resource to
+  //   perform the requested operation. The client may retry after a delay.
   google.rpc.Status status = 1;
 
   // The uri from the request that resulted in a successful retrieval, or from