profiler: Download API returns error when all nodes fail (#6525)

When download profiling data API fails to gather profiling data
from all nodes for any reason (including profiler not enabled),
return 400 http code with the appropriate json message.

cmd/admin-handlers.go

@@ -355,8 +355,7 @@ func (a adminAPIHandlers) DownloadProfilingHandler(w http.ResponseWriter, r *htt
 		return
 	}
 
-	// Return 200 OK
-	w.WriteHeader(http.StatusOK)
+	profilingDataFound := false
 
 	// Initialize a zip writer which will provide a zipped content
 	// of profiling data of all nodes
@@ -371,6 +370,8 @@ func (a adminAPIHandlers) DownloadProfilingHandler(w http.ResponseWriter, r *htt
 			continue
 		}
 
+		profilingDataFound = true
+
 		// Send profiling data to zip as file
 		header, err := zip.FileInfoHeader(dummyFileInfo{
 			name:    fmt.Sprintf("profiling-%d", i),
@@ -391,6 +392,11 @@ func (a adminAPIHandlers) DownloadProfilingHandler(w http.ResponseWriter, r *htt
 			return
 		}
 	}
+
+	if !profilingDataFound {
+		writeErrorResponseJSON(w, ErrAdminProfilerNotEnabled, r.URL)
+		return
+	}
 }
 
 // extractHealInitParams - Validates params for heal init API.