[MM-31512] Downgrade 4xx and 501 errors to info/debug level (#16585)

* Downgrade 4xx and 501 errors to info/debug level

* use switch instead

web/handlers.go

@@ -268,7 +268,15 @@ func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		if c.Err.Id == "api.context.session_expired.app_error" {
 			c.LogInfo(c.Err)
 		} else {
-			c.LogError(c.Err)
+			code := c.Err.StatusCode
+			switch {
+			case code >= http.StatusBadRequest && code < http.StatusInternalServerError:
+				c.LogDebug(c.Err)
+			case code == http.StatusNotImplemented:
+				c.LogInfo(c.Err)
+			default:
+				c.LogError(c.Err)
+			}
 		}
 
 		c.Err.Where = r.URL.Path