Exception when refresh token expires

[jonathanstegall]

Is your feature request related to a problem? Please describe.
It is very rare, but there can be an exception that occurs when the Salesforce refresh token expires before it can be updated. In salesforce.php at line 625:

$refresh_token = $this->get_refresh_token();
if ( empty( $refresh_token ) ) {
	throw new Object_Sync_Sf_Exception( esc_html__( 'There is no refresh token.', 'object-sync-for-salesforce' ) );
}

Describe the solution you'd like
We should figure out a way to make it fix itself rather than throwing this exception.