Dashboard admin notices, show errors for log4php

If the `API_CON_MNGR_LOG_ENABLE` switch is enabled in [index.php](https://github.com/daithi-coombes/api-connection-manager/blob/master/index.php#L17) and there are errors writing to the log file, then show the errors as `admin_notices` in the dashboard.

Use [API_Con_Mngr_Error](https://github.com/daithi-coombes/api-connection-manager/blob/master/class-api-con-mngr-error.php) to add to the `API_Con admin_notices` error message stack