Do not log an exception if side pane was not found (#2791)

Some older versions had different names for the side pane components and their preferred sizes are still associated with these old names. Thus opening a newer JabRef always logged some errors, which only can be resolved by resetting the preferences. 
This PR changes the log level from error to debug since I don't think that the size of a side pane is that essential.

src/main/java/org/jabref/gui/SidePaneManager.java

@@ -167,11 +167,11 @@ private static Map<Class<? extends SidePaneComponent>, Integer> getPreferredPosi
                 Class<? extends SidePaneComponent> componentClass = (Class<? extends SidePaneComponent>) Class.forName(componentName);
                 preferredPositions.put(componentClass, Integer.parseInt(componentPositions.get(i)));
             } catch (ClassNotFoundException e) {
-                LOGGER.error("Following side pane could not be found: " + componentName, e);
+                LOGGER.debug("Following side pane could not be found: " + componentName, e);
             } catch (ClassCastException e) {
-                LOGGER.error("Following Class is no side pane: '" + componentName, e);
+                LOGGER.debug("Following Class is no side pane: '" + componentName, e);
             } catch (NumberFormatException e) {
-                LOGGER.info("Invalid number format for side pane component '" + componentName + "'.", e);
+                LOGGER.debug("Invalid number format for side pane component '" + componentName + "'.", e);
             }
         }